Using clap to generate shell completions

[erickpires]

I thought the shell completions would be very useful in 'loop'. And since the project already uses 'clap' it was very easy to add a 'completions' subcommand to generate them the same way 'rustup' does.

[Miserlou]

Huge and rad! But will this conflict with #19, which is a pretty major rewrite?

[erickpires]

Since the clap::App is not exposed in #19 this is indeed a conflict. I don't know structopt, but since it's a wrapper around clap there should be a way to do the same thing. I will wait for #19 to be merged and then I will try to implement completions.

[Miserlou]

Okay, I have merged 19!

[Miserlou]

Ready for merge?

[erickpires]

I was able to make the completions work with structopt but I stumbled upon a panic due to some Option::unwrap() generated by #[derive(StructOpt)]. I've found no documentation about this, so I posted an issue. I'm currently waiting for some answer.

I was able to workaround this, though. But I think the workaround is very ugly.