Return a non-zero exit code when matches exist

This is just to get the conversation started around some code to solve #34.

Just as an example, [rails_best_practices returns the number of errors found as the exit code][1]. But [reek doesn't][2].
I don't feel strongly one way or the other. But if you're not using the exit codes for anything else, why not? It could be useful.

Also, should this really need a flag to work? Shouldn't this just be the default behavior of the executable?

[1]: https://github.com/railsbp/rails_best_practices/blob/0a44a455d57075dfe8b31a1f15d5b1857b166a84/lib/rails_best_practices/command.rb#L138

[2]: https://github.com/troessner/reek/blob/f0c2a9e0e330795b43e3bf08d19f474ccb8f2f98/lib/reek/cli/application.rb#L13-L15