Add 'woocommerce_ajax_order_item' hook 

The `'woocommerce_ajax_add_order_item_meta'` @maxrice added is good for adding order item meta, but it's still not possible for an extension to edit the `$item` passed to the `order-item-html.php`.

When there is a free trial and no sign up fee, Subscriptions needs to set the `$item`'s line totals to $0. This filter makes that possible.