Refactor of check_link_to to improve code quality

Hi, 

I really love Brakeman (tried to build one myself a couple of years ago, but unfortunately stopped because of time issues), but I saw you where having some code quality issues: https://codeclimate.com/github/presidentbeef/brakeman/Brakeman::CheckLinkTo#duplication . I'd be more then willing to help with this. I've started by reformatting one of the worst offenders (F).

All tests succeed and I tried not to change *any* business logic while refactoring.

 - Broke check_argument into smaller functions
 - Removed warning code duplication
 - Added some one liners to clarify logic