Custom parsing subroutines

Since PGE just converts regexes into parrot subs

and calls to subrules are just subroutine calls

we can write custom subrules as Parrot subs.

• A regex is just another type of subroutine. (Or method.)

• We aren't limited to regex syntax for parsing.

--> We can define custom parse subroutines in any language, and call them from a regex.