Add download file hook.

Allow developers to hook into the download file process.
In my case, I need to stamp PDFs with licensee informations before download.

The hook synopsis:
```php
public function myDownloadFile(
    \FilesModel $objFileModel,
    \Isotope\Model\Download $objDownload,
    \Isotope\Model\ProductCollectionDownload $objCollectionDownload
);
```