Add a -q/--quiet flag

[eklitzke]

I find the default warning message about empty reports to be annoying. This adds a -q/--quiet flag that will suppress the warning log level.

I'm not 100% this is correct because with this change wrapper_environment() still creates ENVIRONMENT_KEY with a "verbose" key that does not consider the value of args.quiet. I'm open to handling this another way too, the way the argument handling and log setting works seemed kind of weird to me.

[rizsotto]

Thanks to look into it. Will think how would it be more silent. Currently this mimics the Clang scan-build command about the output directory handling and reporting. Also a few tests relies on the current behavior.