RFC: Allow silencing warnings (and perhaps errors)

[MestreLion]

As pointed out by @t0mm1e in #52, --quiet does not silence warnings, and there's no way to do so (apart from 2>/dev/null, which would also silence errors.

We have a few options here:

• Change --quiet to also silence warnings. Is this desired behavior? Should it also silence errors? Is it worth breaking current behavior?
• Add a new option, such as --silent or --mute, to extend silence to warnings. This is the approach taken by, IIRC, curl or wget. What is the preferred name? Should it also silence errors? If not, should a 3rd option do so?

I'll leave this one open for user feedback and comments on the best way to implement this.