Add a simple jekyll plugin to replace relative URLs in links with the full URL.

Original source: http://www.slightlytallerthanaverageman.com/2010/02/22/jekyll-feedburner-and-global-urls/#comment-599796216
Extend the plugin to fix image src paths too.
Remove `class` too.
Remove any `data-` attributes too.
Merge pull request #28 from mpc-hc/rss-valid

Make RSS feed valid