Support for dynamic tab completion

[softprops]

Looking for something akin to the dynamic options feature of the golang cli package

https://github.com/alecthomas/kingpin/blob/master/README.md#additional-api

More specifically I would like an alternative to the existing clap API that let's you configure a static array of arg option values that would let me instead provide a fn type that yields a dynamic list.

Bonus points if this could factor into claps current support for tab completion

I would be interested in working on an implementation with some coaching on what the key areas in clap would need to change.

[kbknapp]

This is covered by #1232 and #568

It's something I very much want, I just have some other priorities to get 3.x-beta out the door first 😉 I'm going to close this in favor of the other two issues. Feel free to discuss there and we can work on implementation!

[softprops]

No prob. These are exactly what I'm looking for!