Added Hoedown as an extension to test

Sundown is the base for Redcarpet, which is used in production Github. But Sundown became frozen a while ago, and no changes have been accepted since then.

[Hoedown](https://github.com/hoedown/hoedown) is the revived fork of Sundown. We've fixed bugs, standarized the API, added option parsing, and a lot more!

This pull request makes it possible to test Hoedown. The `hoedown` executable must be on path for it to work. It works pretty fast (on my computer it's the fastest to test, 0.3s) which is natural since it's written in C.