Move rouge loading
Extract plugin management into its own class.
Require 'plugin_manager' in lib/jekyll.rb
Strip content of the {% highlight %} block

A fix for #1801
Improve feed.xml.

* add `generator` property
* add image property
* use `xml_escape` instead of `![CDATA]`
* use `date_to_rfc822` instead of the date format string (thanks @troyswanson!)
* fix self reference url
* switch to `isPermaLink` for the `guid`
Add meta generator property in the HTML files.
STRIP ALL THE NEWLINES FROM HIGHLIGHTED CODE ok just the ones at the beginning and end
Merge pull request #2192 from XhmikosR/docs-feed
Update history to reflect merge of #2192 [ci skip]
Expect 41 posts, not just 40.
Merge pull request #1823 from jekyll/highlight-without-newline
Update history to reflect merge of #1823 [ci skip]
Fetching/determining the plugin path is beyond this it's kind of sad.
Ok handle the plugins_path stuff in the PluginManager.
Merge pull request #2197 from jekyll/plugin-manager
Update history to reflect merge of #2197 [ci skip]
Merge pull request #2189 from benhanzl/move_rouge_loading
Update history to reflect merge of #2189 [ci skip]