Add new Style/ExactRegexpMatch cop

[koic]

This PR adds new Style/ExactRegexpMatch cop that checks for exact regexp match inside Regexp literals.

# bad
string =~ /\Astring\z/
string === /\Astring\z/
string.match(/\Astring\z/)
string.match?(/\Astring\z/)

# good
string == 'string'

# bad
string !~ /\Astring\z/

# good
string != 'string'

---

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.

[bbatsov]

Looks good to me. Thanks!