Switch from marked to faster parser written in C (sundown): Robotskirt

[przemoc]

Currently blacksmith uses marked, which is fine and relatively fast. But there is a faster one, Robotskirt based on the parser written in C: sundown, the one internally used by GitHub. As marked by default uses GitHub Flavored Markdown, it should be possible to perform the switch transparently I guess.

The only problem atm is that Robotskirt is in the process of making some changes to upgrade to recent sundown version (see Issue 16) and do some other improvements (see last comments of Issue 11), thus such switch possibly shouldn't happen before.

[mmalecki]

We're not going to switch to a compiled module because that makes installation harder. We try to avoid compiled modules whenever possible.
Also, marked is relatively fast.