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/siteJSONEncode_test.go

@@ -1,4 +1,4 @@
-// Copyright 2015 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.
@@ -14,12 +14,7 @@
 package hugolib
 
 import (
-	"encoding/json"
 	"testing"
-
-	"path/filepath"
-
-	"github.com/gohugoio/hugo/deps"
 )
 
 // Issue #1123
@@ -27,27 +22,22 @@ import (
 // May be smart to run with: -timeout 4000ms
 func TestEncodePage(t *testing.T) {
 	t.Parallel()
-	cfg, fs := newTestCfg()
 
-	writeSource(t, fs, filepath.Join("content", "page.md"), `---
-title: Simple
+	templ := `{{ index .Site.RegularPages 0 | jsonify }}`
+
+	b := newTestSitesBuilder(t)
+	b.WithSimpleConfigFile().WithTemplatesAdded("index.html", templ)
+	b.WithContent("page.md", `---
+title: "Page"
+date: 2019-02-28
 ---
-Summary text
 
-<!--more-->
+Content.
+
 `)
 
-	s := buildSingleSite(t, deps.DepsCfg{Fs: fs, Cfg: cfg}, BuildCfg{})
+	b.Build(BuildCfg{})
 
-	_, err := json.Marshal(s)
-	check(t, err)
+	b.AssertFileContent("public/index.html", `"Date":"2019-02-28T00:00:00Z"`)
 
-	_, err = json.Marshal(s.RegularPages[0])
-	check(t, err)
-}
-
-func check(t *testing.T, err error) {
-	if err != nil {
-		t.Fatalf("Failed %s", err)
-	}
 }