changed the semantics of required and default in settings definitions.

I think it makes more logical sense now. Essentially, the default is
always used if the user doesn't specify a value. If there is no user
specified value and the default is None, then the action depends on the
value of required. required=True means raise an error. required=False
means silently omit that setting.

overviewer_core/configParser.py

@@ -33,7 +33,7 @@ class MultiWorldParser(object):
 
             self._settings[settingname] = setting
             
-            if not setting.required:
+            if setting.required and setting.default is not None:
                 self._config_state[settingname] = setting.default
 
     def set_config_item(self, itemname, itemvalue):