Add Message extra for permanent URL

Allow bots to easily refer to stream&topic that a message
was part of.

Closes zulip#6

zulip.py

@@ -243,6 +243,13 @@ def _handle_message(self, message):
                 room=room
             )
             message_instance.to = room
+            message_instance.extras['url'] = (
+                self.identity['site'] + '/#narrow/stream/{stream}'
+                '/subject/{subject}/near/{mid}').format(
+                    stream=quote(room.title),
+                    subject=quote(room.subject),
+                    mid=message['id'],
+            )
         else:
             raise ValueError("Invalid message type `{}`.".format(message['type']))
         self.callback_message(message_instance)