Teach Performance/{Start, End}With cops to look for Regexp#match?

[bdewater]

As I found out for fastruby/fast-ruby#150 match? can be just as fast for small strings, but as the string size grows the performance gap widens.

• Wrote good commit messages.
• Feature branch is up-to-date with master (if not - rebase it).
• 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.

[bbatsov]

👍