Abort gracefully when interrupted with Ctrl-C

Currently, `rubocop` dumps dozens of backtrace lines when interrupted with Ctrl-C.

This PR includes following changes, much like RSpec and Cucumber.

* Does not dump backtraces
* Exits with status `1`
* With Ctrl-C once
  * Aborts at time of finishing each file
  * Reports summary
* With Ctrl-C twice
  * Aborts immediately
  * Does not report summary

P.S. Thanks for mentining Guard::Rubocop :)