[Feature request] Add shell command completion autogeneration

[yermulnik]

fswitch already has around a dozen+ of command line arguments which is complicate to remember and operate by humans when necessary.
Would be great to look up and employ a Golang library to autogenerate shell command completions from code on the fly (e.g. tswitch --completion bash) that can be soaked into shell dynamically (e.g. . <(tfswitch --completion bash)).

Common shells that shell command completion is shipped for usually are: Bash, Zsh, and Fish.

Once implemented, update documentation with manual steps and Homebrew formulae to autogenerate and install shell command completions into common location.
Caveat: only @warrensbox has write access to https://github.com/warrensbox/homebrew-tap