Prompt for missing options

[felixSchl]

Explore the idea of adding a [prompt] tag to an option, indicating that if the option has been omitted from the command line and no environment variable substitution has been found, it will prompt the user if at a TTY.

I don't imagine that be too easy, though, since all of neodoc's processing is happening in pure code and adding these side effects will need to happen somewhere in the impure entry module.

Actually, I feel like this might not even be neodoc turf. It should be easy enough to implement this on user-level using some other library or a neodoc companion library maybe.

[felixSchl]

This is never going to happen