Enable friendly permalinks.

Set up "friendly" permalinks in the style of
"/category/some-post-title". Uses a slightly altered version of Will
Norris's jekyll-clean-urls plugin
(https://github.com/willnorris/jekyll-clean-urls) that produces
extensionless files that can be served directly a la Pretzel
(https://github.com/Code52/pretzel). Also adds a simple .htaccess file
with a DefaultType directive so extensionless HTML files can be served
unambiguously.