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, 4 insertions, 4 deletions

diff --git a/create/content.go b/create/content.go
index 31b7b2e4d..264a0f3ac 100644
--- a/create/content.go
+++ b/create/content.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.
@@ -50,7 +50,7 @@ func NewContent(
 
 	if isDir {
 
-		langFs := hugofs.NewLanguageFs(s.Language.Lang, sites.LanguageSet(), archetypeFs)
+		langFs := hugofs.NewLanguageFs(s.Language().Lang, sites.LanguageSet(), archetypeFs)
 
 		cm, err := mapArcheTypeDir(ps, langFs, archetypeFilename)
 		if err != nil {
@@ -113,7 +113,7 @@ func NewContent(
 
 func targetSite(sites *hugolib.HugoSites, fi *hugofs.LanguageFileInfo) *hugolib.Site {
 	for _, s := range sites.Sites {
-		if fi.Lang() == s.Language.Lang {
+		if fi.Lang() == s.Language().Lang {
 			return s
 		}
 	}
@@ -245,7 +245,7 @@ func resolveContentPath(sites *hugolib.HugoSites, fs afero.Fs, targetPath string
 
 	// Try the filename: my-post.en.md
 	for _, ss := range sites.Sites {
-		if strings.Contains(targetPath, "."+ss.Language.Lang+".") {
+		if strings.Contains(targetPath, "."+ss.Language().Lang+".") {
 			s = ss
 			break
 		}