Enable arg converters

[LachlanMarnham]

Command line args are all passed as strings. There should be a way to register converters for a target. Without converters:

@registry.register_target
def target_with_params(param):
    print('param is a string')

Prototype design:

@registry.register_target(converters={'param': int})
def target_with_params(param):
    print('param is an int')

with param = int(param) being called behind the scenes, but an arbitrary converter callback should be acceptable.

[LachlanMarnham]

Currently blocked by #6 and #7