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 #5074Fixes#5763Fixes#5758Fixes#5090Fixes#5204Fixes#4695Fixes#5607Fixes#5707Fixes#5719Fixes#3113Fixes#5706Fixes#5767Fixes#5723Fixes#5769Fixes#5770Fixes#5771Fixes#5759Fixes#5776Fixes#5777Fixes#5778
This is in heavy use in rendering, so this makes a difference:
```bash
benchmark old ns/op new ns/op delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 124551144 107743429 -13.49%
benchmark old allocs new allocs delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 528684 435118 -17.70%
benchmark old bytes new bytes delta
BenchmarkSiteBuilding/TOML,num_langs=1,num_pages=500,tags_per_page=5,shortcodes,render-4 53306848 45147832 -15.31%
```
As per the referenced issue, if the task list in Markdown has
nothing before it, it will be rendered wrongly:
```
---
title: "My First Post"
date: 2017-07-29T20:21:57+02:00
draft: true
---
* [ ] TaskList
```
is rendered as:
```
<ul> class="task-list"
<li><input type="checkbox" disabled class="task-list-item"> TaskList</li>
</ul>
```
The problem lies in the `List` function of `HugoHTMLRenderer`, it had
a hardocded index of `4` for the first `>` of the list, it is used to
insert the class into the text before the closing bracket, but that
hardcoded index is only right when there is a newline before the
opening bracket, which is the case when there is anything in the
document before the task list, but if there is nothing, then there is
no newline, and the correct index of the first `>` will be `3`.
To fix that we're changing the hardcoded index to be dynamic by using
`bytes.Index` to find it properly. We're also adding a test case to
make sure this is tested against.
Fixes#3710
All config variables starts with low-case and uses camelCase.
If there is abbreviation at the beginning of the name, the whole
abbreviation will be written in low-case.
If there is abbreviation at the end of the name, the
whole abbreviation will be written in upper-case.
For example, rssURI.
Inserts a code tag into Pygments output with the language-info that is present when using client-side highlighting (useful for CSS hooks)
```html
<code class="language-go" data-lang="go">
```
closes#1490