Provide support for re.fullmatch in String trait type

[mdickinson]

A minor feature request: for String validation with a regex, re.fullmatch is more often going to be what we want to use than re.match. It would be nice to provide a way to do that full match.

[mdickinson]

Actually, the needs might be better served by a way to plug in a general validation function, or at least documentation of how to easily subclass the Str trait type to add such validation.