Add basic autocompletion for Zsh

[knight42]

Unsolved problem:

Should I hard code this file into somewhere and generate it in a proper place, say ~/.cargo/_rustup(then ask users to copy it to /usr/share/zsh/site-functions),

or simply let rustup copy this file to ~/.cargo/_rustup?

[knight42]

part of #278

[Diggsey]

I'm a little concerned about keeping this up to date with the CLI, but we can add it for now on the basis that slightly out-of-date completions are better than no completions, and our CLI aims to be backwards compatible.

However, I'd prefer not to clutter the root directory with this. src/rustup-cli/zsh/_rustup should be fine, and we should link to it from the README under installation instructions.

Regarding actual installation, for now just providing them in the repo is better than nothing. When we figure out the best way to install it automatically, we can always include! the completions file into the rustup binary.

[knight42]

@Diggsey updated as you said :)

[Diggsey]

Thanks! Test failure looks spurious, so merging this.