Treat accepted values for one-of:s as regex

[jonasandero]

Proposal: It would be nice if one could use regex as values in the last argument to ARG_TYPE_GROUP_SET.

While at it, another option is to add yet another argument - the name of a validation function that would be invoked or a statement that would be evaluated.

[matejak]

This is somehow trickier than the static set of one-of values, consider e.g. bash completion. Could you please share us the use case for this feature?