Add support for Kramdown as Markdown engine

I've been having all sorts of trouble with Redcarpet lately, so I thought I'd take a shot at getting a couple of Middleman's coolest Redcarpet-specific features working with Kramdown.  Here's a patch for Pygments syntax highlighting.

(Yes, I know that Kramdown ships with support for Coderay, but it lacks parsers for many of the languages I need on my site, and this patch makes the Redcarpet-to-Kramdown transition one that doesn't lose any features or make me have to rework my stylesheets.)