New Cop: UnusedImplicitSubject

[marcandre]

Before submitting the PR make sure the following are checked:

• Feature branch is up-to-date with master (if not - rebase it).
• Squashed related commits together.
• Added tests.
• Updated documentation.
• Added an entry to the CHANGELOG.md if the new code introduces user-observable changes.
• The build (bundle exec rake) passes (be sure to run this locally, since it may produce updated documentation that you will need to commit).

If you have created a new cop:

• Added the new cop to config/default.yml.
• The cop is configured as Enabled: pending in config/default.yml.
• The cop documents examples of good and bad code.
• The tests assert both that bad code is reported and that good code is not reported.
• Set VersionAdded in default/config.yml to the next minor version.

[marcandre]

Hmmm. I now realize it contradicts RSpec/ImplicitSubject and maybe I should have modified that instead.

I had read quickly that cop, but got confused by the fact that it deals with two things that aren't related in my mind:

• use of it_is_expected vs expect(subject)
• number of lines / expectactions / whatever