Make Page an interface

The main motivation of this commit is to add a `page.Page` interface to replace the very file-oriented `hugolib.Page` struct.
This is all a preparation step for issue  #5074, "pages from other data sources".

But this also fixes a set of annoying limitations, especially related to custom output formats, and shortcodes.

Most notable changes:

* The inner content of shortcodes using the `{{%` as the outer-most delimiter will now be sent to the content renderer, e.g. Blackfriday.
  This means that any markdown will partake in the global ToC and footnote context etc.
* The Custom Output formats are now "fully virtualized". This removes many of the current limitations.
* The taxonomy list type now has a reference to the `Page` object.
  This improves the taxonomy template `.Title` situation and make common template constructs much simpler.

See #5074
Fixes #5763
Fixes #5758
Fixes #5090
Fixes #5204
Fixes #4695
Fixes #5607
Fixes #5707
Fixes #5719
Fixes #3113
Fixes #5706
Fixes #5767
Fixes #5723
Fixes #5769
Fixes #5770
Fixes #5771
Fixes #5759
Fixes #5776
Fixes #5777
Fixes #5778

1 files changed, 23 insertions, 2 deletions

diff --git a/helpers/general_test.go b/helpers/general_test.go
index 1279df439..ed4c3d2c2 100644
--- a/helpers/general_test.go
+++ b/helpers/general_test.go
@@ -1,4 +1,4 @@
-// Copyright 2018 The Hugo Authors. All rights reserved.
+// 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.
@@ -42,7 +42,7 @@ func TestGuessType(t *testing.T) {
 		{"html", "html"},
 		{"htm", "html"},
 		{"org", "org"},
-		{"excel", "unknown"},
+		{"excel", ""},
 	} {
 		result := GuessType(this.in)
 		if result != this.expect {
@@ -166,6 +166,27 @@ var containsAdditionalTestData = []struct {
 	{"", []byte(""), false},
 }
 
+func TestSliceToLower(t *testing.T) {
+	t.Parallel()
+	tests := []struct {
+		value    []string
+		expected []string
+	}{
+		{[]string{"a", "b", "c"}, []string{"a", "b", "c"}},
+		{[]string{"a", "B", "c"}, []string{"a", "b", "c"}},
+		{[]string{"A", "B", "C"}, []string{"a", "b", "c"}},
+	}
+
+	for _, test := range tests {
+		res := SliceToLower(test.value)
+		for i, val := range res {
+			if val != test.expected[i] {
+				t.Errorf("Case mismatch. Expected %s, got %s", test.expected[i], res[i])
+			}
+		}
+	}
+}
+
 func TestReaderContains(t *testing.T) {
 	for i, this := range append(containsBenchTestData, containsAdditionalTestData...) {
 		result := ReaderContains(strings.NewReader(this.v1), this.v2)