Add a command to just run policies

We should consider adding a command that just runs policies, e.g., verify. It should furthermore offer options like --fail-on-warning as well as have options to control the output format, such as --json to ensure the results can be machine readable.

It should have an option to include successful policies that have been run (if not the default).

An alternative approach would be to enrich the diagnostic messages in preview --json's output so that they carry information like severity level, policy pack, etc.