Add a switch for quiet mode, to silence output such as stderr diagnostics

[lornajane]

Is your feature request related to a problem? Please describe.

From the discussion in #1277 it seems that in some settings, such as in CI where the output won't be seen or checked unless there is a problem, a "quiet" switch to suppress all non-error content directed to stderr would be a good feature to have.

Describe the solution you'd like

Add a --quiet switch (no shorthand needed) to suppress information and warning output about the command to stderr. For commands that do output actual error information such as lint, continue to output information, warnings and other information to stdout as is currently in use.

Describe alternatives you've considered

Keep the current situation: the content of stderr need not be checked for errors if the command exited successfully.