Make cop names more flexible in CommentConfig lines

[tjwp]

#3201 introduced support for arbitrary comments after cop names.

However, the previous regexp for cop names was roughly [\w/]+. The new regexp for a cop name is
([A-Z][a-z]+/)?(?:[A-Z][a-z]+)+.

This regexp is too restrictive for some existing custom cops. All of the cops in rubocop-rspec are in a RSpec department that does not match this regexp.

This change makes the cop name regexp ([A-Z]\w+/)?(?:[A-Z]\w+) to allow better back compatibility with existing custom cops.

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.

[tjwp]

Rebased

[tjwp]

@bbatsov Is there any plan for a 0.41.2 release with this change and #3248?