cli: Allow specifying multiple component or target values in one argument

[jplatte]

It has happened a few times to me now that I've tried to install a toolchain with rustup toolchain install <toolchain> -c a,b,c, only to get an error that the component "a,b,c" does not exist.

This PR changes the meaning of that command to the (IMO obvious) one of installing the toolchain <toolchain> with components a, b and c. Targets can also be specified in one -t argument when delimited with commas.

[kinnison]

Nominally this change is good, however if we want to guarantee to support this going into the future then we need a test to demonstrate the functionality. Could you please sort a test out (probably in cli-v2 or cli-misc) and push it to the PR?

[jplatte]

I've added tests for rustup, should I create additional ones for rustup-init?

[kinnison]

This looks good, However as you note, there are changes in setup_mode.rs which do need verifying too.

Please try and a test to confirm the behaviour in the installer.

[jplatte]

@kinnison Done.