Description:
------------
It would be very helpful to allow transformToDoc to return a custom class instance, which would have to extend DOMDocument of course.

The example code provided is simplified.  Far better usage could be made from the child DOMDocument class -- this is only an example to illustrate the potential.

Test script:
---------------
class DOMDocumentYouCanFancyPrint extends DOMDocument {
    public function __toString() {
        return "<pre>" . htmlentities( $this->saveXML() ) . "</pre>";
    }
}

...

$processor->registerDocumentClass( "DOMDocumentYouCanFancyPrint" );
echo $processor->transformToDoc( $sourceDOM );

Expected result:
----------------
The expected result would be that the transformToDoc method call returns an instance of DOMDocumentYouCanFancyPrint.