Add an option to redirect offence reports to stderr

[knu]

This adds --stderr to make it easier to use --stdin in shell scripts, editor integration, CI workflow, etc.

In response to feedbacks, I altered this PR from changing the behavior of --stdin to adding a new option.

Here's the original description.

This makes editor integration much easier to implement by eliminating the need to process the output.

In the short term, existing editor plugins that expect ==================== will be broken, but it should be relatively easy to fix.

For example, vscode-ruby-rubocop can check if stderr output contains (or ends with) ==================== to see if it needs to delete warnings from stdout output. (to support any version of rubocop)

Eventually, they just need to simply use the stdout output.

---

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 (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.
• The PR relates to only one subject with a clear title and description in grammatically correct, complete sentences.
• Run bundle exec rake default. It executes all tests and RuboCop for itself, and generates the documentation.

[bbatsov]

I get the rationale, but won't this break the existing editor plugins?

[dvandersluis]

Yeah my concern is editors that you can’t just update by pulling a repo are going to be broken until they can be updated (like rubymine).

[knu]

An easiest fix is to add 2>&1 to the command line, and we can inform that or submit PRs to the developers of those known plugins.

https://docs.rubocop.org/rubocop/integration_with_other_tools.html

[knu]

Would it better to keep the behavior and add a new option, like --stderr for example?

[bbatsov]

I think so. Might also be a good idea to invite some of the plugin authors to share their perspective on this.

[knu]

OK, I changed so. A new option --stderr redirects all output except for the autocorrected source to stderr.

[tas50]

Thanks for changing this to be an option. That'll make things a lot easier for those of us that parse rubocop output in editor plugins / other tools

[bbatsov]

I think you should add some section to the docs explaining the use-case for this option, otherwise I doubt people will make use of it.

[knu]

I updated the description of this option mentioning it is useful when combined with --auto-correct and --stdin.

[bbatsov]

Thanks!