Add subcommands to "rustup show" for active-toolchain and it's version

[segevfiner]

This should be useful to shell scripts (via command substitution) and other programs that need this information.

Do note that the new commands currently succeed and print nothing when there is no active tool chain. I'm not sure if that's the proper behavior.

Related: #450

[Diggsey]

Thanks for the PR!

I'm not sure the "version" variant is strictly necessary? You can just run rustc --version. Getting the active toolchain name is potentially useful, although I think there is potential for misusing it to figure out the toolchain directory: the directory should be obtained using rustc --print sysroot.

Could you remove the version variant, and add a note to the help text for the other variant to consider using rustc --print sysroot instead?

[Diggsey]

Also, could you add a test for the new behaviour. Nevermind, I see you've already got one :)