Add GNU extensions to regex. This is done
as one massive review, but I'm more than happy to
split this up into several if that's desired.
The additions to engine.c are not guarded by LIBREGEX
because it looks cleaner and functionally, they'll have
no effect due to the opcodes not being added by non-LIBREGEX
build. If it seems like they may cause speed concerns, I'm
more than happy to LIBREGEX guard them.