feat(cli): add --quiet to rustup toolchain list and friends

[rami3l]

Closes #3519.

[djc]

Nice!

It feels like we should more tightly model verbosity as an enum internally (along the lines of enum Verbosity { Default, Quiet, Verbose }) so we can be more intentional about how to apply it.

[rami3l]

It feels like we should more tightly model verbosity as an enum internally (along the lines of enum Verbosity { Default, Quiet, Verbose }) so we can be more intentional about how to apply it.

@djc I agree. However for the moment being we only have less than 5 instances of --quiets and --verboses in the codebase each, and they don't even form a pair in some places, so I'm a bit hesitant. I'd like to change that when the pattern becomes more visible, but for now I'm inclined to merge this as-is.

[djc]

I ran into https://github.com/rust-lang/rustup/blob/master/src/cli/common.rs#L171 where the interaction of verbose and quiet in the NotifyOnConsole/DownloadTracker (which themselves suffer from crappy design) doesn't seem very well-defined, that's why I was thinking about it.