New cop FlipFlop

Detect the use of flip flops in code.

Flip flops are described in http://batsov.com/articles/2013/09/05/the-elements-of-style-in-ruby-number-10-in-cryptic-perlisms-we-do-not-trust/ , and deprecated in the Ruby Style Guide

No configuration treating the two types of flip-flops differently is provided. Nor is there an EnforcedStyle to make it mandatory. :smile: