Cache "static" includes to improve build performance (#1874)

* Add jekyll-include-cache plugin
* Cache "static" includes to improve build performance
* Update CHANGELOG and history

"Static" refers to those includes that don't rely on data passed from the page content.

docs/_config.yml

@@ -284,6 +284,7 @@ plugins:
   - jekyll-gist
   - jekyll-feed
   - jemoji
+  - jekyll-include-cache
 
 # mimic GitHub Pages with --safe
 whitelist:
@@ -292,6 +293,7 @@ whitelist:
   - jekyll-gist
   - jekyll-feed
   - jemoji
+  - jekyll-include-cache
 
 
 # Archives

docs/_docs/18-history.md

@@ -4,7 +4,7 @@ permalink: /docs/history/
 excerpt: "Change log of enhancements and bug fixes made to the theme."
 sidebar:
   nav: docs
-last_modified_at: 2018-10-02T11:03:51-04:00
+last_modified_at: 2018-10-04T15:41:33-04:00
 toc: true
 ---
 
@@ -12,6 +12,7 @@ toc: true
 
 ### Enhancements
 
+- Cache "static" includes to improve build performance. [#1874](https://github.com/mmistakes/minimal-mistakes/pull/1874)
 - Make entire feature and archive items "clickable". [#1864](https://github.com/mmistakes/minimal-mistakes/pull/1864)
 - Allow custom Staticman endpoints. [#1842](https://github.com/mmistakes/minimal-mistakes/issues/1842)
 - Remove `type="text/css"` from Algolia script includes. [#1836](https://github.com/mmistakes/minimal-mistakes/pull/1836)