Hi,

I've added UUID to thing-at-point.  I think it's a standard enough
format that makes sense to have it in the core. It's my 2nd
contribution, so still getting used to the tools and standards.

The implementation relies on a regular expression to accept the
different versions of uuids (versions 1 to 5), and also the "Nil UUID".

I'm using rx as it makes clearer the intent of what we're parsing at
each point. I hope it's ok to use it (I don't see it used a lot in emacs
itself).

If there are any comments or fixes to do, point them out and I'll make
the changes accordingly.

Thanks,

Raimon Grau