allow for a list of allowed values for an argument

[Ryszard Czerminski]

like choices in argparse (e.g. see: https://stackoverflow.com/questions/15836713/allowing-specific-values-for-an-argparse-argument)

[David Shih]

I don't have the bandwidth for enhancements. Pull requests that are compatible with existing features are welcome.

This seems very easy to do by the client as a post-parsing check.