Should load external content into a page using `source` and trigger `sourceLoaded` event

```
<div data-bind="page: {id: 'lorem', source: 'lorem.html .content', sourceLoaded: loremIsLoaded}"></div>
```

The source can contain a selector (see `.content` above) in order to extract a fragment on the site.
In the example above `loremIsLoaded` is a function that is triggered after `lorem.html` is loaded.
