Update mdast to remark

[wooorm]

Hey @tmcw / mapbox!

I’d like to update mdast to remark. You’ve probably noticed this, but mdast got a big new update including a rename to remark (see what’s new, learn how to upgrade plugins).

I have time to do the update myself if needed, but I’d first like to know whether you’d like to include GH-11, GH-12 first (as the update will probably make rebasing those more difficult).

[tmcw]

👍 done in 1.1.0

[wooorm]

Great work! 👏👏