Actually remove enabled extensions from ignore list

When we enable a plugin (when it's provided in the --enable-extensions)
plugin, we need to remove it both from the extended default ignore list
and the plain ignore list.

Closes #239

src/flake8/plugins/manager.py

@@ -182,10 +182,17 @@ class Plugin(object):
                 LOG.critical(str(failed_to_load))
                 raise failed_to_load
 
-    def enable(self, optmanager):
+    def enable(self, optmanager, options=None):
         """Remove plugin name from the default ignore list."""
         optmanager.remove_from_default_ignore([self.name])
         optmanager.extend_default_select([self.name])
+        if not options:
+            return
+        try:
+            options.ignore.remove(self.name)
+        except (ValueError, KeyError):
+            LOG.debug('Attempted to remove %s from the ignore list but it was '
+                      'not a member of the list.', self.name)
 
     def disable(self, optmanager):
         """Add the plugin name to the default ignore list."""
@@ -202,7 +209,7 @@ class Plugin(object):
                 parse_options(options)
 
         if self.name in options.enable_extensions:
-            self.enable(optmanager)
+            self.enable(optmanager, options)
 
     def register_options(self, optmanager):
         """Register the plugin's command-line options on the OptionManager.

tests/unit/test_plugin.py

@@ -162,3 +162,28 @@ def test_provide_options():
     plugin_obj.parse_options.assert_called_once_with(
         option_manager, option_values, None
     )
+
+
+@pytest.mark.parametrize('ignore_list, code, expected_list', [
+    (['E', 'W', 'F', 'C9'], 'W', ['E', 'F', 'C9']),
+    (['E', 'W', 'F'], 'C9', ['E', 'W', 'F']),
+])
+def test_enable(ignore_list, code, expected_list):
+    """Verify that enabling a plugin removes it from the ignore list."""
+    options = mock.Mock(ignore=ignore_list)
+    optmanager = mock.Mock()
+    plugin = manager.Plugin(code, mock.Mock())
+
+    plugin.enable(optmanager, options)
+
+    assert options.ignore == expected_list
+
+
+def test_enable_without_providing_parsed_options():
+    """Verify that enabling a plugin removes it from the ignore list."""
+    optmanager = mock.Mock()
+    plugin = manager.Plugin('U4', mock.Mock())
+
+    plugin.enable(optmanager)
+
+    optmanager.remove_from_default_ignore.assert_called_once_with(['U4'])