This hook allows plugins to inspect or modify tiddlers before they are saved via the ''confirm'' toolbar button; the hook is not invoked for tiddlers that are saved through other means, such as state tiddlers created by the ActionSetFieldWidget.
The original tiddler object can be returned unmodified by the hook. If the hook needs to modify the tiddler then it should return a new tiddler object, for example:
```
return new $tw.Tiddler(tiddler,{"my-field": value});
```
Hooks must not change the ''title'' field but can freely modify any other field of the tiddler.
