Toggle table of contents via front matter (#1310)

* Add jekyll-toc include
* Reduce whitespace generated by comments
* Add table of contents include to `single` layout
* Replace `toc` include with jekyll-toc enabled YAML Front Matter
* Update README
* Update table of contents documentation
  - Revise `toc` helper include to mention that it will be deprecated in the next major version.
  - Add documentation to `single` layout explaining how to enable table of contents on those pages.
* Update CHANGELOG and history
* Update LICENSE

Close #1222

test/_posts/2012-01-03-layout-table-of-contents-post.md

@@ -5,27 +5,25 @@ header:
   caption: "Photo credit: [**Unsplash**](https://unsplash.com)"
 tags:
   - table of contents
+toc: true
+toc_label: "Unique Title"
+toc_icon: "heart"
 ---
 
-{% include toc title="Unique Title" icon="file-text" %}
+Enable table of contents on post or page by adding `toc: true` to its YAML Front Matter. The title and icon can also be changed with:
 
-Testing Kramdown auto-generated table of contents with unique title and icon assigned in the include like so:
-
-```liquid
-{% raw %}{% include toc title="Unique Title" icon="file-text" %}{% endraw %}
+```yaml
+---
+toc: true
+toc_label: "Unique Title"
+toc_icon: "heart"  # corresponding Font Awesome icon name (without fa prefix)
+---
 ```
 
 ## HTML Elements
 
 Below is just about everything you'll need to style in the theme. Check the source code to see the many embedded elements within paragraphs.
 
-<h1>Heading 1</h1>
-<h2>Heading 2</h2>
-<h3>Heading 3</h3>
-<h4>Heading 4</h4>
-<h5>Heading 5</h5>
-<h6>Heading 6</h6>
-
 ## Body text
 
 Lorem ipsum dolor sit amet, test link adipiscing elit. **This is strong**. Nullam dignissim convallis est. Quisque aliquam.