Cmdline option for less verbose output (--silent/--quiet)

[amit1rrr]

Currently we print verbose output which shows every test that's running. It's a good default since,

• Users can see the progress (some tests in notebooks can be long running)
• Easy to make sure all intended tests have run

But verbose output is not always required. Users would sometimes prefer py.test type output which only becomes loud on failure and is silent otherwise. It would be good to have a flag (--quiet/--silent) to only print bare minimum output.