[Fix #9122] Added notification if any gems are loaded that have rubocop extensions.

[dvandersluis]

The notification is output at the end, and only when using a few specified formatters that make sense to me. It is also disabled for CI and if some rubocop options are set that I didn't think it made sense to be included for (--only, --debug, --list-target-files and --out (since this is possibly used for something programmatic, I didn't want to change it).

The message looks like this:

Notice: Based on detected gems, the following RuboCop extension libraries might be helpful:
  * rubocop-rake (http://github.com/rubocop-hq/rubocop-rake)

This notification can be disabled by adding this to your config:
  AllCops:
    NotifyForExtensions: false

I also wanted to have a way to disable it so that it doesn't end up being spammy to users who aren't interested, so it is configured by AllCops/NotifyForExtensions (which can either take a hash of extensions and gems, or false). I did it this way so that someone could disable the notification for just rubocop-rake for instance, if they're never planning on using it.

I would love to discuss any choices I made and make any changes!

---

Before submitting the PR make sure the following are checked:

• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• 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.
• Ran bundle exec rake default. It executes all tests and runs RuboCop on its own code.
• Added an entry (file) to the changelog folder named {change_type}_{change_description}.md if the new code introduces user-observable changes. See changelog entry format for details.

[dvandersluis]

/cc @bbatsov @tejasbubane

[bbatsov]

I love the proposed design and I might even ship this with the release I've planned for later today, as I don't really have any concerns about it. It's actually better than what I had in mind, as originally I was thinking of mapping extensions to the presence of certain files/folders in the project. You came up with a granular and flexible configuration and I agree that something like this doesn't make sense for every formatter.

I'm assuming in the unlikely event of a project with only a .gemspec we might run into some issues, but that's not a big deal. As far as I'm concerned all we need is some documentation is this is pure gold. Thanks! 🙇

[dvandersluis]

@bbatsov thanks! My biggest worry is how to make this be useful without being spammy.

I'll write up some documentation for it in the morning, thanks for the reminder!

[dvandersluis]

@bbatsov oh I was writing tests and documentation 😅 I'll open another PR