Add progress bar to offense formatter

[drewpterry]

When you run bundle exec rubocop --format offenses there is no output until rubocop has run completely. It is not very user friendly when run over large codebases where you might go a few minutes before rubocop shows any sign that it is indeed properly running.

This PR simply adds a progress bar so the user knows rubocop is running.

---

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• Wrote good commit messages.
• Commit message starts with [Fix #issue-number] (if the related issue exists).
• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Added an entry to the Changelog if the new code introduces user-observable changes. See changelog entry format.
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  and description in grammatically correct, complete sentences.
• Run rake default or rake parallel. It executes all tests and RuboCop for itself, and generates the documentation.

[pocke]

When you run bundle exec rubocop --format offenses there is no output until rubocop has run completely. It is not very user friendly when run over large codebases where you might go a few minutes before rubocop shows any sign that it is indeed properly running.

Definitely.

This PR simply adds a progress bar so the user knows rubocop is running.

Yeah, it's good idea. But I think the progress-bar should be hidden if the output is not a tty. I guess we would like to use the formatter output as an input of some program sometimes. For example: $ rubocop --format offenses | ruby -e 'do_something STDIN.read'
In this case, we do not need the progress-bar.

[drewpterry]

@pocke

But I think the progress-bar should be hidden if the output is not a tty.

That makes a lot of sense. I made the requested change!

[bbatsov]

Thanks for working on this! I really appreciate it! 🙇