Make column counter variable available in filter

This commit simply makes the existing column counter variable (`$i`) available as a parameter within the `edd_download_class` filter hook. This is beneficial in cases where you want to apply specific styles to downloads in the grid based on which column they are in or their order in the list.

Example usage:

```php
function my_edd_download_class( $class, $id, $atts, $i ) {
	if ( 0 === $i % 2 ) {
		$class .= ' edd_download_even';
	}
	return $class;
}
add_filter( 'edd_download_class', 'my_edd_download_class', 10, 4 );
```