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

hugolib/embedded_shortcodes_test.go

@@ -1,4 +1,4 @@
-// Copyright 2016 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.
@@ -20,6 +20,8 @@ import (
 	"strings"
 	"testing"
 
+	"github.com/spf13/cast"
+
 	"path/filepath"
 
 	"github.com/gohugoio/hugo/deps"
@@ -67,9 +69,11 @@ func doTestShortcodeCrossrefs(t *testing.T, relative bool) {
 
 	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
 
-	require.Len(t, s.RegularPages, 1)
+	require.Len(t, s.RegularPages(), 1)
 
-	output := string(s.RegularPages[0].content())
+	content, err := s.RegularPages()[0].Content()
+	require.NoError(t, err)
+	output := cast.ToString(content)
 
 	if !strings.Contains(output, expected) {
 		t.Errorf("Got\n%q\nExpected\n%q", output, expected)