Initializer hard-codes parsers list, hard to extend

[jaredcwhite]

It would be awesome if the GFM parser could be extended more easily…as it is, the initializer hard-codes span parsers (and maybe block parsers…hard to tell), so there's no way to extend without monkey-patching, or subclassing (but then anything which is configured specifically to use GFM wouldn't load the subclass).

For example, I wanted to add <mark> highlighting via :: or == which is common in many Markdown editors. I had to monkey-patch in order to do so, which isn't the end of the world, but it'd be nice to have a more extensible solution. Thoughts?