add --silent parameter to silence summary output

Adds a `--silent` flag, which makes the report omit summary data from output.

For my use-case I'm writing a git integration gem to provide git hooks that reject commits that have newly-authored or edited lines with rubocop errors, which will make gradual healing of large projects possible. Omitting the summary output makes the output significantly simpler to parse when run with the `--emacs` flag.

I considered adding `--[no-]summary` instead, but communicating the default behavior in the `--help` output, and supporting a default of true, proved to make things more complex. I'm open to suggestions on what to call the flag if `--silent` doesn't seem like a good fit.