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, 6 insertions, 10 deletions

diff --git a/hugolib/multilingual.go b/hugolib/multilingual.go
index c09e3667e..6f744f3a5 100644
--- a/hugolib/multilingual.go
+++ b/hugolib/multilingual.go
@@ -1,4 +1,4 @@
-// Copyright 2016-present 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.
@@ -62,10 +62,10 @@ func newMultiLingualFromSites(cfg config.Provider, sites ...*Site) (*Multilingua
 	languages := make(langs.Languages, len(sites))
 
 	for i, s := range sites {
-		if s.Language == nil {
-			return nil, errors.New("Missing language for site")
+		if s.language == nil {
+			return nil, errors.New("missing language for site")
 		}
-		languages[i] = s.Language
+		languages[i] = s.language
 	}
 
 	defaultLang := cfg.GetString("defaultContentLanguage")
@@ -78,19 +78,15 @@ func newMultiLingualFromSites(cfg config.Provider, sites ...*Site) (*Multilingua
 
 }
 
-func newMultiLingualForLanguage(language *langs.Language) *Multilingual {
-	languages := langs.Languages{language}
-	return &Multilingual{Languages: languages, DefaultLang: language}
-}
 func (ml *Multilingual) enabled() bool {
 	return len(ml.Languages) > 1
 }
 
 func (s *Site) multilingualEnabled() bool {
-	if s.owner == nil {
+	if s.h == nil {
 		return false
 	}
-	return s.owner.multilingual != nil && s.owner.multilingual.enabled()
+	return s.h.multilingual != nil && s.h.multilingual.enabled()
 }
 
 func toSortedLanguages(cfg config.Provider, l map[string]interface{}) (langs.Languages, error) {