Support for emphasized text less than 3 characters

[PeterTillema]

Right now, _ab_ outputs literally _ab_ instead of ab, while _abc_ outputs (correctly) abc.

[mattallty]

This seems like a limitation/bug of the marked dependency.
https://www.npmjs.com/package/marked

[PeterTillema]

https://marked.js.org/demo/?text=_ab_ this looks fine to me.