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

resources/page/pages_prev_next.go

@@ -0,0 +1,42 @@
+// 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.
+// You may obtain a copy of the License at
+// http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+package page
+
+// Prev returns the previous page reletive to the given
+func (p Pages) Prev(cur Page) Page {
+	for x, c := range p {
+		if c.Eq(cur) {
+			if x == 0 {
+				// TODO(bep) consider return nil here to get it line with the other Prevs
+				return p[len(p)-1]
+			}
+			return p[x-1]
+		}
+	}
+	return nil
+}
+
+// Next returns the next page reletive to the given
+func (p Pages) Next(cur Page) Page {
+	for x, c := range p {
+		if c.Eq(cur) {
+			if x < len(p)-1 {
+				return p[x+1]
+			}
+			// TODO(bep) consider return nil here to get it line with the other Nexts
+			return p[0]
+		}
+	}
+	return nil
+}