Suport struc tag "choices"

[davidxjohnson]

I've looked at quite a few command-line and configuration packages. My selection criteria - in order of preference - is:

1. to focus on the use-case of writing GitHub actions (which rely on env vars). I don't need a configuration package that reads from a file, and accepts flags, and env vars - simpler is better for my needs.
2. to support a "choices" tag to validate the env var. There is only one package out there that does this (that I could find) but doesn't use struc tags to do it.
3. to find something relatively simple and well written that I can modify to do what I need.

After looking at the code in this project, I think it meets all my basic needs - except the "choices" struc tag - and doesn't look very hard to modify. I will fork this project and create a pull request when I have something complete, tested and documented.