Structure handlers to allow .get() to return lists

See #1154. In the end I didn't modify the Attributehandler and
TagHandler like this, instead I added the `return_list` argument
for cases when one wants a guaranteed return.

evennia/scripts/monitorhandler.py

@@ -93,6 +93,7 @@ class MonitorHandler(object):
     def at_update(self, obj, fieldname):
         """
         Called by the field as it saves.
+
         """
         to_delete = []
         if obj in self.monitors and fieldname in self.monitors[obj]:
@@ -175,6 +176,9 @@ class MonitorHandler(object):
         """
         List all monitors.
 
+        Returns:
+            monitors (list): The handled monitors.
+
         """
         output = []
         for obj in self.monitors: