Split out from #294, Flow Comments are a path to nice, terse docs that don't require preprocessing and would run in node natively. Right now Babel doesn't parse Flow Comments.

Ideally this is implemented upstream

https://phabricator.babeljs.io/T2038

https://github.com/babel-plugins/babel-plugin-flow-comments turns annotations into comments, we want the opposite.

gajus/eslint-plugin-flowtype#87 is blocked by the same issue.