[#5451]: When using --auto-gen-config, do not ouput offenses unless the --output-offenses flag is also passed

[drewpterry]

#5451

As mentioned in the issue running rubocop with the --auto-gen-config option currently displays all offenses in the console, which is not particularly useful for this feature, especially when being run over large code bases.

This makes it so that --auto-gen-config will not output offenses to the
console by default.

Additionally, a flag of --output-offenses is also added which can be
used in conjuction with the auto-gen-config command to output offenses
to the console if needed.

---

Before submitting the PR make sure the following are checked:

• 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.
• The PR relates to only one subject with a clear title
  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.

[Drenmi]

Good call using a custom formatter. Looks reasonable to me.

[bbatsov]

I think there was also some documentation of the formatters in the manual, that should probably be updated.

I'd also name --output-offenses to --display-offenses, as it seems like a better name to me. In general I'm not quite certain that we need this option at all, but I guess doesn't hurt to have it.

[drewpterry]

@Drenmi @bbatsov

Thanks for taking a look!

I decided to remove the --output-offenses option completely since I was unsure of including it initially as well and I don't think it's worth adding extra code for it unless some other users explicitly ask for it.

Additionally I added some documentation in the formatters manual as suggested.

[bbatsov]

👍