Properly allow escaped `\.` on enumerations to disable them

[jlevy]

Need a way to disable enumarations, e.g. paragraphs that start with 1. blah blah but should not be enumerated. A 1\. blah blah should do this.

See https://github.com/jlevy/atom-flowmark/commit/8883e8cb67b078fe81d1b2ce03a8c28d41f3039b — Flowmark currently removes that backslash! :(

[wooorm]

It’s a remark problem :'(

There is code for this, but for some reason it doesn’t work: https://github.com/remarkjs/remark/blob/e8e65ceaeacf96ab8fe63bbb43fc36c70d08254a/packages/remark-stringify/lib/escape.js#L120-L142