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/sitemap_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.
@@ -18,10 +18,10 @@ import (
 
 	"reflect"
 
-	"github.com/stretchr/testify/require"
-
+	"github.com/gohugoio/hugo/config"
 	"github.com/gohugoio/hugo/deps"
 	"github.com/gohugoio/hugo/tpl"
+	"github.com/stretchr/testify/require"
 )
 
 const sitemapTemplate = `<urlset xmlns="http://www.sitemaps.org/schemas/sitemap/0.9">
@@ -86,14 +86,14 @@ func doTestSitemapOutput(t *testing.T, internal bool) {
 
 func TestParseSitemap(t *testing.T) {
 	t.Parallel()
-	expected := Sitemap{Priority: 3.0, Filename: "doo.xml", ChangeFreq: "3"}
+	expected := config.Sitemap{Priority: 3.0, Filename: "doo.xml", ChangeFreq: "3"}
 	input := map[string]interface{}{
 		"changefreq": "3",
 		"priority":   3.0,
 		"filename":   "doo.xml",
 		"unknown":    "ignore",
 	}
-	result := parseSitemap(input)
+	result := config.DecodeSitemap(config.Sitemap{}, input)
 
 	if !reflect.DeepEqual(expected, result) {
 		t.Errorf("Got \n%v expected \n%v", result, expected)