Add --verbose flag, obey rule of silence of it's not passed

[srenatus]

Hey there.

I'd like to work on a small PR that makeselvis rock output nothing (but exit 0) if there's no violation found, and output the rule violations only if there are any.

Executing elvis --verbose rock would then make elvis behave like it does today

I expect that this proposal needs some discussion, and I could be convinced that a --quiet flag would be preferable (to not change the default). However, my favourite behaviour would be no-problem-no-output.

Any thoughts?

[elbrujohalcon]

I like your original idea, as long as, if there are warnings they're printed out as they're today.

[srenatus]

😃 Yay!

I've noticed that there already is a no_output flag in elvis_core, but it's not set by elvis. However, it's suppressing all output, which isn't what we'd want here.

[srenatus]

I'd assumed that #448's "Fixes..." comment would close this automatically... anyhow, thanks for merging those PRs, I'm looking forward to the next elvis release 😉