Add set_event argument to .shutdown() method

powerline/__init__.py

@@ -726,10 +726,19 @@ class Powerline(object):
 				pass
 			yield FailedUnicode(safe_unicode(e))
 
-	def shutdown(self):
-		'''Shut down all background threads. Must be run only prior to exiting 
-		current application.
+	def shutdown(self, set_event=True):
+		'''Shut down all background threads.
+
+		:param bool set_event:
+			Set ``shutdown_event`` and call ``renderer.shutdown`` which should 
+			shut down all threads. Set it to False unless you are exiting an 
+			application.
+
+			If set to False this does nothing more then resolving reference 
+			cycle ``powerline → config_loader → bound methods → powerline`` by 
+			unsubscribing from config_loader events.
 		'''
+		if set_event:
 			self.shutdown_event.set()
 			try:
 				self.renderer.shutdown()