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

config/services/servicesConfig.go

@@ -1,4 +1,4 @@
-// Copyright 2018 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.
@@ -23,6 +23,7 @@ const (
 
 	disqusShortnameKey = "disqusshortname"
 	googleAnalyticsKey = "googleanalytics"
+	rssLimitKey        = "rssLimit"
 )
 
 // Config is a privacy configuration for all the relevant services in Hugo.
@@ -31,6 +32,7 @@ type Config struct {
 	GoogleAnalytics GoogleAnalytics
 	Instagram       Instagram
 	Twitter         Twitter
+	RSS             RSS
 }
 
 // Disqus holds the functional configuration settings related to the Disqus template.
@@ -61,6 +63,12 @@ type Twitter struct {
 	DisableInlineCSS bool
 }
 
+// RSS holds the functional configuration settings related to the RSS feeds.
+type RSS struct {
+	// Limit the number of pages.
+	Limit int
+}
+
 // DecodeConfig creates a services Config from a given Hugo configuration.
 func DecodeConfig(cfg config.Provider) (c Config, err error) {
 	m := cfg.GetStringMap(servicesConfigKey)
@@ -76,5 +84,9 @@ func DecodeConfig(cfg config.Provider) (c Config, err error) {
 		c.Disqus.Shortname = cfg.GetString(disqusShortnameKey)
 	}
 
+	if c.RSS.Limit == 0 {
+		c.RSS.Limit = cfg.GetInt(rssLimitKey)
+	}
+
 	return
 }