Added Unicode category support

[cmkilger]

I added support for Unicode categories, as listed on this site, e.g. \p{P} for punctuation or \p{Cherokee} for Cherokee characters. I'm not sure how you would actually want to do this, so I just went for it. It doesn't verify to make sure that the value used and just assumes it is available. And it uses some heuristics to try and format it properly.

If you'd rather, all possible values could be added to the grammar, and then there could be a big map to the descriptions of each one.

[javallone]

Thank you for this change.

As far as I can tell, Unicode categories aren't supported by JavaScript-style regular expressions (which is all that Regexper supports at this time), so I'm not going to merge this right now. I will keep this change around though for when other regex styles are added to the site.

[cmkilger]

I assume you mean just native RegExp, but I'll just point out that XRegExp brings support for this to JavaScript. Perhaps that would be considered another style.