Allow extra attributes to be added to offsite links via a filter

With this change, additional attributes can be added to the offsite links via a filter. Example usage:

Make links open in new window:
```php
function my_external_product_link_attrs( $attributes ) {
	return $attributes . ' target="_blank"';
}
add_filter( 'edd_external_product_link_attrs', 'my_external_product_link_attrs' );
```

Add "nofollow" to all external product links:
```php
function my_external_product_link_attrs( $attributes ) {
	return $attributes . ' rel="nofollow"';
}
add_filter( 'edd_external_product_link_attrs', 'my_external_product_link_attrs' );
```