Human readable severity-based output format

[hamishwillee]

The default json output is great if you're a GCS and you can reformat the information as appropriate for users.

As a human though it is pretty dense for scanning. It would be great to be able to get output that shows just the fail cases/warnings (ie omits success cases). More generically, perhaps to be able to specify output by "severity".

In addition, sorting by cumulative severity and then individual severity would make it easier for a human to immediately determine the most important problems.