Added CONF.is_digipi

This will help Craig with custom code changes that might be
needed only on the digipi.  He likes to modify the UI for
webchat.  So in the new webchat extension simply add a call
if CONF.is_digipi:
  # Do your awesome changes here
else:
  # normal aprsd/webchat stuffs here.

aprsd/conf/common.py

@@ -137,6 +137,12 @@ aprsd_opts = [
         help='Set this to False, to disable sending of ack packets. This will entirely stop'
         'APRSD from sending ack packets.',
     ),
+    cfg.BoolOpt(
+        'is_digipi',
+        default=False,
+        help='Set this to True, if APRSD is running on a Digipi.'
+        'This is useful for changing the behavior of APRSD to work with Digipi.',
+    ),
 ]
 
 watch_list_opts = [