Topic: Add save command. Closes GH-332.

I committed this already earlier, but that commit was lost somewhere.
It doesn't seem to be in any branch, I suspect I might have reset my
testing branch or something later, or I don't know.

plugins/Topic/plugin.py

@@ -573,6 +573,24 @@ class Topic(callbacks.Plugin):
         self._sendTopics(irc, channel, topics)
     swap = wrap(swap, ['canChangeTopic', 'topicNumber', 'topicNumber'])
 
+    @internationalizeDocstring
+    def save(self, irc, msg, args, channel):
+        """[<channel>]
+
+        Saves the topic in <channel> to be restored with @topic default
+        later. <channel> is only necessary if the message isn't sent in
+        the channel itself.
+        """
+        if not self._checkManageCapabilities(irc, msg, channel):
+            capabilities = self.registryValue('requireManageCapability')
+            irc.errorNoCapability(capabilities, Raise=True)
+        topic = irc.state.getTopic(channel)
+        if topic:
+            self.setRegistryValue('default', value=topic, channel=channel)
+        else:
+            self.setRegistryValue('default', value='', channel=channel)
+    save = wrap(save, ['canChangeTopic'])
+
     @internationalizeDocstring
     def default(self, irc, msg, args, channel):
         """[<channel>]