Add Goldmark as the new default markdown handler

This commit adds the fast and CommonMark compliant Goldmark as the new default markdown handler in Hugo.

If you want to continue using BlackFriday as the default for md/markdown extensions, you can use this configuration:

```toml
[markup]
defaultMarkdownHandler="blackfriday"
```

Fixes #5963
Fixes #1778
Fixes #6355

2 files changed, 267 insertions, 0 deletions

diff --git a/markup/tableofcontents/tableofcontents.go b/markup/tableofcontents/tableofcontents.go
new file mode 100644
index 000000000..6cd84e5ae
--- /dev/null
+++ b/markup/tableofcontents/tableofcontents.go
@@ -0,0 +1,148 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tableofcontents
+
+import (
+	"strings"
+)
+
+type Headers []Header
+
+type Header struct {
+	ID   string
+	Text string
+
+	Headers Headers
+}
+
+func (h Header) IsZero() bool {
+	return h.ID == "" && h.Text == ""
+}
+
+type Root struct {
+	Headers Headers
+}
+
+func (toc *Root) AddAt(h Header, y, x int) {
+	for i := len(toc.Headers); i <= y; i++ {
+		toc.Headers = append(toc.Headers, Header{})
+	}
+
+	if x == 0 {
+		toc.Headers[y] = h
+		return
+	}
+
+	header := &toc.Headers[y]
+
+	for i := 1; i < x; i++ {
+		if len(header.Headers) == 0 {
+			header.Headers = append(header.Headers, Header{})
+		}
+		header = &header.Headers[len(header.Headers)-1]
+	}
+	header.Headers = append(header.Headers, h)
+}
+
+func (toc Root) ToHTML(startLevel, stopLevel int) string {
+	b := &tocBuilder{
+		s:          strings.Builder{},
+		h:          toc.Headers,
+		startLevel: startLevel,
+		stopLevel:  stopLevel,
+	}
+	b.Build()
+	return b.s.String()
+}
+
+type tocBuilder struct {
+	s strings.Builder
+	h Headers
+
+	startLevel int
+	stopLevel  int
+}
+
+func (b *tocBuilder) Build() {
+	b.buildHeaders2(b.h)
+}
+
+func (b *tocBuilder) buildHeaders2(h Headers) {
+	b.s.WriteString("<nav id=\"TableOfContents\">")
+	b.buildHeaders(1, 0, b.h)
+	b.s.WriteString("</nav>")
+}
+
+func (b *tocBuilder) buildHeaders(level, indent int, h Headers) {
+	if level < b.startLevel {
+		for _, h := range h {
+			b.buildHeaders(level+1, indent, h.Headers)
+		}
+		return
+	}
+
+	if b.stopLevel != -1 && level > b.stopLevel {
+		return
+	}
+
+	hasChildren := len(h) > 0
+
+	if hasChildren {
+		b.s.WriteString("\n")
+		b.indent(indent + 1)
+		b.s.WriteString("<ul>\n")
+	}
+
+	for _, h := range h {
+		b.buildHeader(level+1, indent+2, h)
+	}
+
+	if hasChildren {
+		b.indent(indent + 1)
+		b.s.WriteString("</ul>")
+		b.s.WriteString("\n")
+		b.indent(indent)
+	}
+
+}
+func (b *tocBuilder) buildHeader(level, indent int, h Header) {
+	b.indent(indent)
+	b.s.WriteString("<li>")
+	if !h.IsZero() {
+		b.s.WriteString("<a href=\"#" + h.ID + "\">" + h.Text + "</a>")
+	}
+	b.buildHeaders(level, indent, h.Headers)
+	b.s.WriteString("</li>\n")
+}
+
+func (b *tocBuilder) indent(n int) {
+	for i := 0; i < n; i++ {
+		b.s.WriteString("  ")
+	}
+}
+
+var DefaultConfig = Config{
+	StartLevel: 2,
+	EndLevel:   3,
+}
+
+type Config struct {
+	// Heading start level to include in the table of contents, starting
+	// at h1 (inclusive).
+	StartLevel int
+
+	// Heading end level, inclusive, to include in the table of contents.
+	// Default is 3, a value of -1 will include everything.
+	EndLevel int
+}
diff --git a/markup/tableofcontents/tableofcontents_test.go b/markup/tableofcontents/tableofcontents_test.go
new file mode 100644
index 000000000..1ea96c82f
--- /dev/null
+++ b/markup/tableofcontents/tableofcontents_test.go
@@ -0,0 +1,119 @@
+// Copyright 2019 The Hugo Authors. All rights reserved.
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package tableofcontents
+
+import (
+	"testing"
+
+	qt "github.com/frankban/quicktest"
+)
+
+func TestToc(t *testing.T) {
+	c := qt.New(t)
+
+	toc := &Root{}
+
+	toc.AddAt(Header{Text: "Header 1", ID: "h1-1"}, 0, 0)
+	toc.AddAt(Header{Text: "1-H2-1", ID: "1-h2-1"}, 0, 1)
+	toc.AddAt(Header{Text: "1-H2-2", ID: "1-h2-2"}, 0, 1)
+	toc.AddAt(Header{Text: "1-H3-1", ID: "1-h2-2"}, 0, 2)
+	toc.AddAt(Header{Text: "Header 2", ID: "h1-2"}, 1, 0)
+
+	got := toc.ToHTML(1, -1)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h1-1">Header 1</a>
+      <ul>
+        <li><a href="#1-h2-1">1-H2-1</a></li>
+        <li><a href="#1-h2-2">1-H2-2</a>
+          <ul>
+            <li><a href="#1-h2-2">1-H3-1</a></li>
+          </ul>
+        </li>
+      </ul>
+    </li>
+    <li><a href="#h1-2">Header 2</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+	got = toc.ToHTML(1, 1)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h1-1">Header 1</a></li>
+    <li><a href="#h1-2">Header 2</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+	got = toc.ToHTML(1, 2)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h1-1">Header 1</a>
+      <ul>
+        <li><a href="#1-h2-1">1-H2-1</a></li>
+        <li><a href="#1-h2-2">1-H2-2</a></li>
+      </ul>
+    </li>
+    <li><a href="#h1-2">Header 2</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+	got = toc.ToHTML(2, 2)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#1-h2-1">1-H2-1</a></li>
+    <li><a href="#1-h2-2">1-H2-2</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+}
+
+func TestTocMissingParent(t *testing.T) {
+	c := qt.New(t)
+
+	toc := &Root{}
+
+	toc.AddAt(Header{Text: "H2", ID: "h2"}, 0, 1)
+	toc.AddAt(Header{Text: "H3", ID: "h3"}, 1, 2)
+	toc.AddAt(Header{Text: "H3", ID: "h3"}, 1, 2)
+
+	got := toc.ToHTML(1, -1)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li>
+      <ul>
+        <li><a href="#h2">H2</a></li>
+      </ul>
+    </li>
+    <li>
+      <ul>
+        <li>
+          <ul>
+            <li><a href="#h3">H3</a></li>
+            <li><a href="#h3">H3</a></li>
+          </ul>
+        </li>
+      </ul>
+    </li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+	got = toc.ToHTML(3, 3)
+	c.Assert(got, qt.Equals, `<nav id="TableOfContents">
+  <ul>
+    <li><a href="#h3">H3</a></li>
+    <li><a href="#h3">H3</a></li>
+  </ul>
+</nav>`, qt.Commentf(got))
+
+}