Add a property for off-by-default plugins

If a plugin is off-by-default use its entry-point name (as we currently
do in flake8 2.x) to add it to the default ignore list.

flake8/plugins/manager.py

@@ -108,6 +108,11 @@ class Plugin(object):
 
         return self._plugin_name
 
+    @property
+    def off_by_default(self):
+        """Return whether the plugin is ignored by default."""
+        return getattr(self.plugin, 'off_by_default', False)
+
     def execute(self, *args, **kwargs):
         r"""Call the plugin with \*args and \*\*kwargs."""
         return self.plugin(*args, **kwargs)  # pylint: disable=not-callable
@@ -181,6 +186,9 @@ class Plugin(object):
             )
             add_options(optmanager)
 
+        if self.off_by_default:
+            optmanager.extend_default_ignore([self.name])
+
 
 class PluginManager(object):  # pylint: disable=too-few-public-methods
     """Find and manage plugins consistently."""