Use kramdown to process markdown

Prefer Kramdown over Redcarpet, reverts e586d8c9eabdc747ab97120fbbf20d7b29465998.

There should no changes to the output, or performance degradations that I can detect since we're just using vanilla markdown now.

The big reason for the push is to allow markdown extra, specifically atributes, as in https://github.com/github/choosealicense.com/pull/90.

/cc @afeld