feature/out of tree checks

These changes allow us to easily host brakeman checks outside the source tree. We have custom checks that aren't generally meaningful and would be cumbersome to others if they were in the source tree.

This is a slight modification to the original pull request (https://github.com/presidentbeef/brakeman/pull/483) on this topic that should fix the travis build. I created a new pull request because merging the previous one up to master was annoying.

Your previous feedback that this code `require`s using absolute paths rather than the current code which uses relative path still holds. My thought was that directories of checks outside brakeman's root dir could be anywhere and using relative paths in that case might be finicky. Thoughts?