consider using remarkable

(this PR is a suggestion/request. Typically I know you would expect a PR like this to be discussed in advance, but I decided to do the PR first to show how minimal the impact would be).

Replaces markdown.js with [remarkable](https://github.com/jonschlinkert/remarkable).

  - remarkable has 100% support for [commonmark](http://jgm.github.io/CommonMark/spec.html)
  - [faster](https://github.com/jonschlinkert/remarkable/tree/master/benchmark)
  - easier to monkeypatch. this is a goal of ours, might be useful to you at some point given the custom tokenizing/parsing you're doing for documentation.