Generate completions with miniserve --print-completions <shell>

[rouge8]

This patch adds a completions subcommand --print-completions option to generate shell completion files at runtime. This ensures the completions are always up to date.

Fixes #377.

CI should pass once #481 is merged. This is some of my first Rust code so lmk if I'm doing anything weird...

[svenstaro]

Hey, thanks a ton! I've been meaning to do this but you just went ahead and did it and that's great! My thoughts are: Shall we make this an option rather than a subcommand? We don't use any subcommands in miniserve so far and I don't really want to change the user flow for this particular thing. I imagine something like --print-completions <shell> and then also restrict it to the available shells for better user feedback.

[rouge8]

Shall we make this an option rather than a subcommand? We don't use any subcommands in miniserve so far and I don't really want to change the user flow for this particular thing. I imagine something like --print-completions

Agreed, done! I got the subcommand idea from #377, but I agree another option makes more sense here.

also restrict it to the available shells for better user feedback.

Yup, that part is done by using the structopt::clap::Shell enum as the option type: https://github.com/svenstaro/miniserve/pull/482/files#diff-c3fd08257cd326dfa1c2a3c0a69e6b842f194950ac32e7b2c6e2750f52d1765fR137. I added a test with an invalid shell option to ensure it fails.

[svenstaro]

Excellent, thanks!