[BUG] macros in invoice number

Hi,

there is a bug in the macro’s for invoice number feature. The problem is that when WordPress is configured with a standard date format different from the ISO one, the macros ( {{year}} for example ) does not work ( the macro return the date 1970-01-01).

The fix is quite easy. In includes/class-woo-invoice-pro-engine.php line 1977, just replace :
$order_created = get_the_date( $d='', $orderId );
by :
$order_created = get_the_date( 'Y-m-d', $orderId );

That way, the strtotime wich will use the $order_created variable will allways work whatsoever the wordpress date format setting done by the wordpress administrator.