shell completions

[peacememories]

Hi, and thanks for this great tool, I don't think we would be able to move our development setup to nix if not for your work.

One point of friction I've found with arion is that it currently does not come with bash completions (at least in the nixpkgs version), which is especially noticeable in contrast with docker, which has pretty excellent completions. I noticed that arion uses optparse-applicative, which seems to be able to autogenerate a completion script, so I'm assuming to get basic command completions we would just have to properly package said script. Even better would be if this also supported fish, because that's the shell I am using right now.

Of course once this works completions could be finetuned as well, but I'd be happy with just having arion restart etc. completed instead of trying to give me arion <some folder starting with re>/ ;)