Enable multiple replies for plugins

This patch adds the ability for plugins to send multiple messages
back in response to a command/message.  The plugin simple needs
to return a list of messages (Strings).  Each string in that list
will result in a separate message being sent back to the originator
of the message.

aprsd/threads.py

@@ -253,6 +253,20 @@ class APRSDRXThread(APRSDThread):
         try:
             results = pm.run(packet)
             for reply in results:
+                if isinstance(reply, list):
+                    # one of the plugins wants to send multiple messages
+                    found_command = True
+                    for subreply in reply:
+                        LOG.debug("Sending '{}'".format(subreply))
+
+                        msg = messaging.TextMessage(
+                            self.config["aprs"]["login"],
+                            fromcall,
+                            subreply,
+                        )
+                        self.msg_queues["tx"].put(msg)
+
+                else:
                     found_command = True
                     # A plugin can return a null message flag which signals
                     # us that they processed the message correctly, but have