Add Common Markdown dialect

[yamikuronue]

Could you add Common Markdown as an option: http://jgm.github.io/stmd/spec.html

One of the applications I use relies on this library for their markdown parsing, and there's been a lot of talk about adhering to the new flavor. It'd be great if we could :)

EDIT: Now that I've done some investigation I see that you have options as to which dialect to build with, so I've amended this to adding the Common dialect