Compatibility with Sequential Orders Pro

I have made changes according to the Seq Order notes. Seems to work! Please add to next release.

https://github.com/skyverge/woocommerce-sequential-order-numbers

= Get the order number =
If you have access to the order ID or order object, you can easily get the sequential order number based on WooCommerce core functions.

$order = wc_get_order( $order_id );
$order_number = $order->get_order_number();