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/_docs/18-history.md

@@ -4,7 +4,8 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2017-10-16T12:40:54-04:00
+last_modified_at: 2017-10-20T14:22:30-04:00
+toc: true
 ---
 
 ## Unreleased
@@ -20,6 +21,7 @@ last_modified_at: 2017-10-16T12:40:54-04:00
 - Add Naver Webmaster Tools verification. [#1286](https://github.com/mmistakes/minimal-mistakes/pull/1286)
 - Add support for Staticman v2 endpoint and reCAPTCHA.
 - Add Polish localized UI text strings. [#1304](https://github.com/mmistakes/minimal-mistakes/pull/1304)
+- Add toggleable table of contents via YAML Front Matter. Note: `toc` helper include will be deprecated in next major version. [#1222](https://github.com/mmistakes/minimal-mistakes/issues/1222) 
 
 ### Bug Fixes