Introduce clean URLs plugin.

Loooong story short, Jekyll's handling of friendly permalinks is a
little naive. See https://github.com/jekyll/jekyll/pull/2294.

The original version of this plugin outputs a file name of
"/some-title.html" instead of the default "/some-title/index.html" that
Jekyll currently produces for a permalink style ending with "/:title".
The idea being to configure the server to serve "some-title.html" when
"some-title" is requested, through Apache Multiviews or similar,
resulting in clean, properly-behaved URLs.

The modified version of this plugin being committed outputs a file name
of "some-file" (no extension) under the same circumstances. It's
appropriate for when the server can be easily configured to serve
extensionless files with a default HTML content type.