Add support for URLs relative to context root

Setting `RelativeURLs` to `true` will make all relative URLs in the site *really* relative.

And will do so with speed.

So:

In `/post/myblogpost.html`:

`/mycss.css` becomes `../mycss.css`

The same in `/index.html` will become:

`./mycss.css` etc.

Note that absolute URLs will not be touched (either external resources, or URLs constructed with `BaseURL`).

The speediness is about the same as before:

```
benchmark                    old ns/op     new ns/op     delta
BenchmarkAbsURL              17462         18164         +4.02%
BenchmarkAbsURLSrcset        18842         19632         +4.19%
BenchmarkXMLAbsURLSrcset     18643         19313         +3.59%
BenchmarkXMLAbsURL           9283          9656          +4.02%

benchmark                    old allocs     new allocs     delta
BenchmarkAbsURL              24             28             +16.67%
BenchmarkAbsURLSrcset        29             32             +10.34%
BenchmarkXMLAbsURLSrcset     27             30             +11.11%
BenchmarkXMLAbsURL           12             14             +16.67%

benchmark                    old bytes     new bytes     delta
BenchmarkAbsURL              3154          3404          +7.93%
BenchmarkAbsURLSrcset        2376          2573          +8.29%
BenchmarkXMLAbsURLSrcset     2569          2763          +7.55%
BenchmarkXMLAbsURL           1888          1998          +5.83%

```

Fixes #1104
Fixes #622
Fixes #937
Fixes #157

1 files changed, 32 insertions, 21 deletions

diff --git a/transform/chain_test.go b/transform/chain_test.go
index cd6dfcde9..808c20121 100644
--- a/transform/chain_test.go
+++ b/transform/chain_test.go
@@ -3,6 +3,7 @@ package transform
 import (
 	"bytes"
 	"github.com/spf13/hugo/helpers"
+	"path/filepath"
 	"strings"
 	"testing"
 )
@@ -62,6 +63,11 @@ schemaless: <img srcset='//img.jpg' src='//basic.jpg'>
 schemaless2: <img srcset="//img.jpg" src="//basic.jpg2> POST
 `
 
+const REL_PATH_VARIATIONS = `PRE. a href="/img/small.jpg" POST.`
+const REL_PATH_VARIATIONS_CORRECT = `PRE. a href="../../img/small.jpg" POST.`
+
+const testBaseURL = "http://base/"
+
 var abs_url_bench_tests = []test{
 	{H5_JS_CONTENT_DOUBLE_QUOTE, CORRECT_OUTPUT_SRC_HREF_DQ},
 	{H5_JS_CONTENT_SINGLE_QUOTE, CORRECT_OUTPUT_SRC_HREF_SQ},
@@ -85,11 +91,13 @@ var srcset_xml_tests = []test{
 	{SRCSET_XML_SINGLE_QUOTE, SRCSET_XML_SINGLE_QUOTE_CORRECT},
 	{SRCSET_XML_VARIATIONS, SRCSET_XML_VARIATIONS_CORRECT}}
 
+var relurl_tests = []test{{REL_PATH_VARIATIONS, REL_PATH_VARIATIONS_CORRECT}}
+
 func TestChainZeroTransformers(t *testing.T) {
 	tr := NewChain()
 	in := new(bytes.Buffer)
 	out := new(bytes.Buffer)
-	if err := tr.Apply(in, out); err != nil {
+	if err := tr.Apply(in, out, []byte("")); err != nil {
 		t.Errorf("A zero transformer chain returned an error.")
 	}
 }
@@ -112,7 +120,7 @@ func TestChaingMultipleTransformers(t *testing.T) {
 	tr := NewChain(f1, f2, f3, f4)
 
 	out := new(bytes.Buffer)
-	if err := tr.Apply(out, helpers.StringToReader("Test: f4 f3 f1 f2 f1 The End.")); err != nil {
+	if err := tr.Apply(out, helpers.StringToReader("Test: f4 f3 f1 f2 f1 The End."), []byte("")); err != nil {
 		t.Errorf("Multi transformer chain returned an error: %s", err)
 	}
 
@@ -124,8 +132,7 @@ func TestChaingMultipleTransformers(t *testing.T) {
 }
 
 func BenchmarkAbsURL(b *testing.B) {
-	absURL, _ := absURLFromURL("http://base")
-	tr := NewChain(absURL...)
+	tr := NewChain(AbsURL)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -134,8 +141,7 @@ func BenchmarkAbsURL(b *testing.B) {
 }
 
 func BenchmarkAbsURLSrcset(b *testing.B) {
-	absURL, _ := absURLFromURL("http://base")
-	tr := NewChain(absURL...)
+	tr := NewChain(AbsURL)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -144,8 +150,7 @@ func BenchmarkAbsURLSrcset(b *testing.B) {
 }
 
 func BenchmarkXMLAbsURLSrcset(b *testing.B) {
-	absXMLURL, _ := absURLInXMLFromURL("http://base")
-	tr := NewChain(absXMLURL...)
+	tr := NewChain(AbsURLInXML)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -154,31 +159,33 @@ func BenchmarkXMLAbsURLSrcset(b *testing.B) {
 }
 
 func TestAbsURL(t *testing.T) {
-	absURL, _ := absURLFromURL("http://base")
-	tr := NewChain(absURL...)
+	tr := NewChain(AbsURL)
 
 	apply(t.Errorf, tr, abs_url_tests)
 
 }
 
+func TestRelativeURL(t *testing.T) {
+	tr := NewChain(AbsURL)
+
+	applyWithPath(t.Errorf, tr, relurl_tests, helpers.GetDottedRelativePath(filepath.FromSlash("/post/sub/")))
+
+}
+
 func TestAbsURLSrcSet(t *testing.T) {
-	absURL, _ := absURLFromURL("http://base")
-	tr := NewChain(absURL...)
+	tr := NewChain(AbsURL)
 
 	apply(t.Errorf, tr, srcset_tests)
 }
 
 func TestAbsXMLURLSrcSet(t *testing.T) {
-	absURLInXML, _ := absURLInXMLFromURL("http://base")
-	tr := NewChain(absURLInXML...)
+	tr := NewChain(AbsURLInXML)
 
 	apply(t.Errorf, tr, srcset_xml_tests)
-
 }
 
 func BenchmarkXMLAbsURL(b *testing.B) {
-	absURLInXML, _ := absURLInXMLFromURL("http://base")
-	tr := NewChain(absURLInXML...)
+	tr := NewChain(AbsURLInXML)
 
 	b.ResetTimer()
 	for i := 0; i < b.N; i++ {
@@ -187,17 +194,17 @@ func BenchmarkXMLAbsURL(b *testing.B) {
 }
 
 func TestXMLAbsURL(t *testing.T) {
-	absURLInXML, _ := absURLInXMLFromURL("http://base")
-	tr := NewChain(absURLInXML...)
+	tr := NewChain(AbsURLInXML)
 	apply(t.Errorf, tr, xml_abs_url_tests)
 }
 
 type errorf func(string, ...interface{})
 
-func apply(ef errorf, tr chain, tests []test) {
+func applyWithPath(ef errorf, tr chain, tests []test, path string) {
 	for _, test := range tests {
 		out := new(bytes.Buffer)
-		err := tr.Apply(out, strings.NewReader(test.content))
+		var err error
+		err = tr.Apply(out, strings.NewReader(test.content), []byte(path))
 		if err != nil {
 			ef("Unexpected error: %s", err)
 		}
@@ -207,6 +214,10 @@ func apply(ef errorf, tr chain, tests []test) {
 	}
 }
 
+func apply(ef errorf, tr chain, tests []test) {
+	applyWithPath(ef, tr, tests, testBaseURL)
+}
+
 type test struct {
 	content  string
 	expected string