Output completions for bash, zsh, etc

[scooter-dangle]

If we have a strong model into which we parse the roff input, it should be easy to peak over clap's shoulder and generate completions for the same set of shells it supports.

[jedugsem]

could this be done though serde and i'll maybe try to use this with redoxs ion shell

[pickfire]

clap already supports completions