Add an option to report errors only in cli output

[elchenberg]

As a user I'd like to suppress the output for files that have no issues:

File:   Dockerfile
Issues: None found 👍

Currently errors and non-errors are printed to stdout. If errors were printed to stderr instead, the non-error output could be redirected to /dev/null.

Or there could be a cli argument – eg. -q, --quiet Report issues only – to achieve the same result.

I'm willing to implement this myself if you approve of the idea. :slightly_smiling_face: