Completions

[kbknapp]

This commit adds a command line option to generate completion scripts, as well as updates clap to 2.16.0 which adds ZSH completion support.

Here's a gif of the new ZSH support and the new rustup completions subcommand backing it.

Specifically this commit adds

$ rustup completions <shell>

Finally, this PR adds some additional help documentation on the completions command, and changes the README.md slightly to reflect the new additions.

Some notes about the ZSH script generated by clap, it supports, subcommands, flags, args, and options. The options also automatically list their possible_values, and will automatically not list any arguments they conflict with. This may not be too applicable to rustup since I don't think many of those options are used, but it's good to note for future reference.

Also, positional arguments, such as toolchain in a few of the subcommands is changed to uppercase ASCII by clap in order to differentiate.

Edit: removed verbiage about --completions

[Diggsey]

This is awesome 👍 My preference would be to stick with just rustup completions, since rustup self commands usually take the form rustup self <subcommand> <args...> rather than just rustup self <args...>

[kbknapp]

That's a simple change - just let me know if you need me to change it, or any other aspects and I'd be happy to do so 😉

[brson]

Pretty amazing @kbknapp. I agree with @Diggsey . Let's just have one way to do it.

[kbknapp]

I've removed the --completions arg. It's now just rustup completions <SHELL>