Implement positional args

[KOLANICH]

No description provided.

[jamesderlin]

dropt is meant to parse options, not positional arguments, but I'll consider adding some separate mechanism to handle positional arguments. Usually handling positional arguments is either straightforward (in which case dropt doesn't have much to offer) or requires specialized rules (in which case dropt would need to provide a lot of flexibility or would be too rigid to be useful).