Allow plugins that are on by default to be ignored

Previously, to ensure that plugins on by default were reported, we
added them to the select list. This means that ignoring them became
impossible. To accomodate our reporting logic and a user's ability
to ignore, we need to keep our select and extended select lists
separated.

This allows us to have a better understanding of who is selecting what,
where, and how and make our decision as to whether or not an error
should be reported more wisely.

Closes #195

tests/unit/test_style_guide.py

@@ -12,6 +12,7 @@ from flake8.plugins import notifier
 def create_options(**kwargs):
     """Create and return an instance of optparse.Values."""
     kwargs.setdefault('select', [])
+    kwargs.setdefault('extended_default_select', [])
     kwargs.setdefault('ignore', [])
     kwargs.setdefault('disable_noqa', False)
     return optparse.Values(kwargs)