Add `groups` argument to Regex filter

[todofixthis]

Often when I use the Regex filter, I'm not interested in capturing the matching groups as a list; I just want to verify that the incoming string matches the regular expression, and then continue applying filters to the original string.

Add a groups argument to Regex that can be set to any of the following:

• True (default, for bc): Filter returns all of the matching groups as a list (this is the current behaviour).
• False: Filter returns the input string.
• Iterable[int]: Filter returns a list containing only the specified groups.
• int (maybe): Filter returns just that group.

That last one feels a bit off, both semantically and functionally. But, it could be useful, so I'll float the idea anyway.

Examples:

>>> value = '42-86-99'

# Default behaviour
>>> Regex('\d+').apply(value)
['42', '86', '99']
>>> Regex('\d+', groups=True).apply(value)
['42', '86', '99']

>>> Regex('\d+', groups=False).apply(value)
'42-86-99'

>>> Regex('(\d)(\d)', groups=(0, 1)).apply(value)
['42', '4']

# Maybe
>>> Regex('(\d)(\d)', groups=1).apply(value)
'4'