multirust update --quiet

I’d like to use `multirust update` in a cron job, where it should not emit any output. My current work around is `sh -c 'multirust update > /dev/null' 2>&1 | grep -v stable`, but a `--quiet` command-line parameter would be nice.

Start by locating the multirust update command and its command-line option handling. Run the command in a cron-like environment to verify the requested quiet behavior; done means multirust update accepts --quiet and emits no output.

Written by the indexing model from the issue text.