add extend in config setting

tiramisu/setting.py

@@ -408,6 +408,10 @@ class Settings(object):
             props.remove(propname)
             self._setproperties(props, None, None)
 
+    def extend(self, propnames):
+        for propname in propnames:
+            self.append(propname)
+
     def _setproperties(self, properties, opt, path):
         """save properties for specified opt
         (never save properties if same has option properties)