add option to show cops disabed by inline comments

Sometimes its tempting for developers, especially juniors who may have trouble understanding some of RuboCop's warnings, to abuse the `# rubocop:disable` functionality. This pull request adds an option (`-x` or `--show-disabled`) that, after a run, prints a list of all the files, line ranges and corresponding cops that were ignored during the run due to `# rubocop:disable` comments.