Adding regexCaptures to GCDWebServerRequest

I found it a little odd that there is regex matching for URLs but there didn't seem to be a simple way to get the captured variables. So I added a new property to GCDWebServerRequest called `regexCaptures` which will now contain those matches.

I tried to adhere to all the style and use of macros, but let me know if something is amiss. I haven't extensively tested all the various things that could go wrong, but from a little testing in my own server it seems to be working nicely.

I didn't update any pod spec details or other versions as that didn't seem appropriate, but I can if you'd like.