Tweak front-matter vs site configuration per comments from @parkr on jekyll/jekyll#2439
Make sure Cleaner doesn't remove dirs if they only contain subdirs

Closes #2204
Fixing help command in deprecation warning
Keep parents of directories in keep_files

Closes #1297
Merge pull request #2457 from marcransome/master
Update history to reflect merge of #2457 [ci skip]
Remove `Command.globs` method

It is no longer being used.
Get relative paths of directories to ignore

Use `Pathname#realpath` to get absolute paths so that
`Pathname#relative_path_from` will not raise an exception. Also add the
config files to this list.
Add jekyll_github_sample plugin
Merge pull request #2463 from bwillis/patch-1
Update history to reflect merge of #2463 [ci skip]
Using implicit returns to conform to coding style
Changes to help clarify defaults in site config vs front-matter
Merge pull request #2458 from karouf/cleaner-empty-dirs
Update history to reflect merge of #2458 [ci skip]
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