Use the `flags.Value` interface for positional argument types

[maxlandon]

Currently, the code parsing the positional argument words (strings) onto their type is directly taken from
the old go-flags code. While not working badly, the range of types supported is largely inferior to the range
of types supported for flags.

This is not really hard, and should not represent a huge workload.
I just realized this would be better toward the end of my work.

• Use the flags.Value type for generating positional arguments slots and parsing words onto them.