Use bindings for cmark-gfm instead

[josevalim]

Folks, what do you think about replacing the bindings for cmark-gfm instead?

It has some extensions, like tables, which we could leave disabled by default. This means we get the commonmark behaviour but there is some extra functionality if someone wants to opt-in?

[asaaki]

Sounds like a great idea.

GFM is Version 0.29-gfm (2019-04-06), but since I haven't updated the C sources for a while I think we shouldn't see any significant differences anyway, the commonmark spec 0.30 just released this summer, but haven't checked the changes, and if any of them would be good to have (meaning: to stick with the regular cmark lib).

Note: source repo for GFM is at https://github.com/github/cmark-gfm

If anyone wants to play with it, y'all more than welcome to open a PR. I might not have the time and capacity to do it myself right now.