Support multiple rubocop:disable comments per line

[vergenzt]

Supporting this would make implementing #4127 much easier because the code wouldn't ever have to add cops to an existing rubocop:disable comment--it could always simply append # rubocop:disable <cop>.

What do you think?

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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).
• Used the same coding conventions as the rest of the project.
• 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.
• All tests are passing.
• The new code doesn't generate RuboCop offenses.
• The PR relates to only one subject with a clear title
  and description in grammatically correct, complete sentences.
• Updated cop documentation with rake generate_cops_documentation (required only when you've added a new cop or changed the configuration/documentation of an existing cop).

[rrosenblum]

I think that I prefer # rubocop:disable Cop1, Cop2 over # rubocop:disable Cop1 # rubocop:disable Cop2.

[vergenzt]

That works. 👍 I'll work out the logic to modify existing statements.