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

docs/_posts/2013-08-16-markup-syntax-highlighting.md

@@ -1,12 +1,13 @@
 ---
 title: "Markup: Syntax Highlighting"
 excerpt: "Post displaying the various ways of highlighting code in Markdown."
-last_modified_at: 2017-07-07T15:32:43-04:00
+last_modified_at: 2017-10-20T12:23:00-04:00
 header:
   teaser: "assets/images/markup-syntax-highlighting-teaser.jpg"
 tags: 
   - code
   - syntax highlighting
+toc: true
 ---
 
 Syntax highlighting is a feature that displays source code, in different colors and fonts according to the category of terms. This feature facilitates writing in a structured language such as a programming language or a markup language as both structures and syntax errors are visually distinct. Highlighting does not affect the meaning of the text itself; it is intended only for human readers.[^1]
@@ -98,4 +99,4 @@ Indentation matters. Be sure the indent of the code block aligns with the first
 
 An example of a Gist embed below.
 
-<script src="https://gist.github.com/mmistakes/77c68fbb07731a456805a7b473f47841.js"></script>
+<script src="https://gist.github.com/mmistakes/77c68fbb07731a456805a7b473f47841.js"></script>