Add support for capturing groups

[RunDevelopment]

Me again :)

I would like to have access to the capturing groups of matches. Ideally, a way to use captures_iter and be exposed.

I would also like to have access to the static capturing group information on rust's regexes. Exposing capture_names should be good enough, but I also wouldn't mind a different API. I just need to know (1) how many capturing groups there are and (2) the mapping of named capturing groups to their indices.

[RunDevelopment]

Thanks @2fd!