Switch from kramdown to redcarpet

For markdown processing, and especially syntax highlighting for fenced code blocks, using pygments. 

I do not like Pygments.rb's additional dependency on Python 2.x. There are many tickets on the pygments and pygments.rb issue tracker that state that Python 3 is supported, and merged pull requests to boot, but the README doesn't promise support and in my experience the support was not there.

So along the way, I tried out CodeRay, but CodeRay's actual highlighting was substandard and not as granular. So, I reverted to Pygments, and that's what I'm shipping. It works well, and I'm using a theme to make it resemble GitHub's theme as closely as possible.