Add -F/--fail-fast option

This is something that I have found useful when working with correcting offenses (in RuboCop itself). I have an external program that monitors file changes in the directory tree and re-runs `./bin/rubocop` when something changes. When I save a file it is the first one checked in the next run, and execution stops at the first file with offenses.

There's some overlap of functionality between [guard-rubocop](https://github.com/yujinakayama/guard-rubocop) and this option, but I just found that adding this functionality inside RuboCop itself solved my problem. I'd like to hear what @yujinakayama thinks, and @bbatsov of course.