Add quiet flag

[layday]

No description provided.

[henryiii]

This line never gets tested, are you thinking of adding a test?

[layday]

Yeah, I haven't added any tests yet.

[layday]

I wonder if we should add two quietness levels, -q and -qq. The first suppresses subprocess (build backend) output, the second suppresses all output. WDYT?

[henryiii]

I like that idea, personally; cibuildwheel will be able to select from multiple levels too. Though I wasn't sure which was more useful to suppress, our output or the build-backend output. Maybe three levels, -q suppresses our output, -qq suppresses the build backend, and -qqq surpasses both? Sort of assuming build backends will always be noisier than our output there. Though I'm fine with the two level approach. Not sure filtering out our output only would ever be needed.