support for `choices` flag

[danieleades]

in addition to showing a user the default value (if it exists) the user should also be shown the available 'choices' when prompted for a missing arg

[multimeric]

Seems like a reasonable request.

[danieleades]

I might get some time to take a look at this. What version of python are you targeting?

[multimeric]

Nothing in particular, since it's so simple, but I'd like to keep as many 3.X versions compatible as possible.

[bwinkers]

PR created for one possible solution.
This blocks the user until they choose a vaid option so it may not be what you wanted.
#7