Support for multi_fetch_fragments

Here's another wish-list item... it'd be great if sync either integrated with or natively did what https://github.com/n8/multi_fetch_fragments/ did. It would be good if the fragment cached actually included the script tags. So you could do something like this:

```haml
  = sync partial: 'list_item', collection: things, scope: scope, cache: true
```

and each cache fragment would include the script tags as well as the view.