Add support for PCRE

[m-ender]

I'm not 100% sure yet whether I want this, but there's a PCRE wrapper for .NET: https://github.com/ltrzesniewski/pcre-net ... by including this in Retina, I could allow switching to PCRE flavour with a regex modifier like e (p I guess), for when you really want recursive patterns or \K.

[m-ender]

I don't think this is going to happen at this point. I'm focusing too much on what .NET's regex flavour can do when designing the language, that probably even using ECMAScript might be problematic at this point.