Add support for look-behinds (?<=ABC)

[jarommadsen]

This is a super cool library and I'm shocked it's not more popular. I can't count the number of times I've had to relearn regex and something like this would help loads in learning and maintaining regex expressions.

I did notice that there isn't any support for look-behinds (?<=ABC) / (?<!ABC). Not sure if there's any motivation to support this library due to the lack of usage but thought I'd reach out and see.

[MaxArt2501]

Thank you for your remarks! The main reason this library isn't popular at all it's because I don't give it enough time :) There are quite some thing I'd like to do with it, starting with a complete rewrite (because, as it is, the code's pretty much unreadable), probably in TypeScript. And, of course, adding support to all the new features of JavaScript's regular expressions.