Enhancement: Add a "resource" parameter to allow retrieving a crop from a child resource.

This would be useful for a listing page to display an image from the child resource it links to.

For example in a pdoResources tpl, you could do something like:
```
<a href="[[~[[+id]]]]">
    <img src="[[!ImageCropper? &resource=`[[+id]]` &image=`[[*crop_img_tpl]]` &field=`preview`]]">
</a>
```