Let the Debugger cop also check for byebug calls

[Byebug](https://github.com/deivid-rodriguez/byebug) is a popular debugger that works with Ruby 2 (which [Debugger](https://github.com/cldwalker/debugger) doesn’t). Would it be ok to add a `byebug` as we’ve done here, or should it rather be a separate cop?