These show that there were some subtle bugs in how we were matching
things and that there are bugs that need to be fixed.

Refs #365

Move all Violation related methods from the StyleGuide to our Violation
class.

Convert it to test the DecisionEngine class directly and put them in
their own file.

In dealing with the decision logic in the StyleGuide recently I
recognized that the logic really doesn't belong strictly on the
StyleGuide. A separate object makes perfect sense especially from the
perspective of testability. This is a minor refactor intended solely to
facilitate further testing and perhaps making the logic easier to
understand for others.

Add test to cover branch of decision logic we were not previously
exercising

There was a *very* subtle bug in how we handle blanket select statements
with error codes that are in our DEFAULT_IGNORE. In the specific case,
users were specifying ``--select E`` and E126 was not being properly
reported. This unveiled further logic bugs as we refined this.

Closes #318

When we refactored our decision process to fix #239, we broke a subtle
corner case where extensions that are not off-by-default are to be
reported.

This further refactors that logic and adds specific tests around it to
ensure it works as expected and doesn't regress.

Closes #257

Previously, we didn't handle the case where an error code was implicitly
ignored (by not being in --select) and implicitly selected (by not being
in --ignore). This means we need to update StyleGuide#_decision_for and
StyleGuide#is_user_selected to handle these cases.

Closes #242
Related-to #239
Related-to !132