Option to silence warnings, automatically silence warnings for non-displayed files

[sgb-io]

Two issues relating to warnings

1. Warnings are useful but sometimes they add noise and may not be able to fix them. For example, you're analyzing a monorepo and receive warnings relating to code that you cannot (or do not want to) fix right now. Also, sometimes, you may not be able to fix a warning for technical reasons. It would be nice to have a --suppress-warnings CLI option to just silence them all.
2. The tool will currently fire warnings for files that are later discarded from output. That's a bit confusing: it should probably only output warnings for files that will be included in the output.