Evaluate type-docopt (type validation w/ docopt)

[bittner]

The original docopt implementation supports default values, but it doesn't support types including choices, let alone type validation. There are two alternatives for that:

• Manual schema validation, see also Enum for option's parameter docopt/docopt#327
• type-docopt, see also Add type and choices to docopt dreamgonfly/type-docopt#1

This topic is also discussed on the docopt-ng issue tracker.