Allow cargo install --version as well (preferred)

[kivikakk]

Fixes #4590.

The usage looks pretty ugly with this extra line in it, but it's not really avoidable with our docopt use.

We fail if both are provided; accepting both feels like a path to demons.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @matklad (or someone else) soon.

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[alexcrichton]

@bors: r+

Thanks!

[bors]

📌 Commit 0cbfb67 has been approved by alexcrichton

[kivikakk]

I think we might need to ask bors to repeat the AppVeyor build; looks like a transient 503 while prepping the build environment.

[bors]

⌛ Testing commit 0cbfb67 with merge 75f8541...

[bors]

☀️ Test successful - status-appveyor, status-travis
Approved by: alexcrichton
Pushing 75f8541 to master...