Refactor decision logic into its own object

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.

tests/unit/test_style_guide.py

@@ -183,22 +183,16 @@ def test_should_report_error(select_list, ignore_list, error_code, expected):
 )
 def test_decision_for_logic(select, ignore, extend_select, enabled_extensions,
                             error_code, expected):
-    """Verify the complicated logic of StyleGuide._decision_for.
-
-    I usually avoid testing private methods, but this one is very important in
-    our conflict resolution work in Flake8.
-    """
-    guide = style_guide.StyleGuide(
+    """Verify the complicated logic of DecisionEngine.decision_for."""
+    decider = style_guide.DecisionEngine(
         create_options(
             select=select, ignore=ignore,
             extended_default_select=extend_select,
             enable_extensions=enabled_extensions,
         ),
-        listener_trie=None,
-        formatter=None,
     )
 
-    assert guide._decision_for(error_code) is expected
+    assert decider.decision_for(error_code) is expected
 
 
 @pytest.mark.parametrize('error_code,physical_line,expected_result', [