Spice up site_template feed
Update explanation of the site front-matter defaults section
Allow Jekyll's logger to be set to any Logger compatible instance

* Stevenson now inherits from Logger and extends some methods to use $stderr
for log messages greater than info level.
* LogWriter provides an interface between Jekyll and Logger to maintain API.
Renaming LogWriter to LogAdapter and adding tests for class

String coloring moved to Stevenson as not responsibility of LogAdapter
Rocket style hash and switching integers for Logger severity constants
Tweak front-matter vs site configuration per comments from @parkr on jekyll/jekyll#2439
Changes to help clarify defaults in site config vs front-matter
If rouge isn't there, say so.

In the previous version, it would say 'redcarpet' wasn't installed,
due to the rescue LoadError block on line 93. This change will tell
the user that, in fact, rouge isn't installed and that this is the
cause of the error, not that redcarpet isn't installed.

Fixes #2464.
https://github.com/jekyll/jekyll/issues/2464
Update history to reflect closing of #2464. [ci skip]
Merge pull request #2438 from pathawks/spicy-feed
Update history to reflect merge of #2438 [ci skip]
Merge pull request #2439 from tschmidt/update-configuration-documentation
Update history to reflect merge of #2439 [ci skip]
Merge pull request #2444 from createdbypete/configurable-logger
Update history to reflect merge of #2444 [ci skip]
Fix link to jekyll-import docs