Highlight regex comments

[DanielOaks]

It would be nice to be able to highlight regex comments explicitly with this, since I'm using this lib to highlight a regex that I split into parts, and indent and comment to explain. Something like the # character perhaps.

Thanks for the awesome library! I might even just do it myself and send a PR, we'll see :)

[slevithan]

Thanks! A PR would be welcome. It could work by passing flag x, since there's an x flag proposal for future versions of JavaScript.