Introduce '--silent' option, fixes #905

This fixes #905 **Implement --silent to only output errors and not warnings**

I initially envisioned this option as a more specific option like `--warn-ignored-files=false`, but finally decided to go with the result of the discussion that already took place in #905 and changed it to `--silent`

I'll be away until next week, so feel free to merge this change, perform additional changes, or just comment and wait for me to be back :)