title: Hook before woocommerce_order_status_completed is fired
body: <p>Is there a hook before woocommerce_order_status_completed is fired? </p>
  
  <p>I have installed a fedex labelling plugin that creates fedex label of a particular order and produces fedex tracking number. What i want to do is to create a condition that checks if the fedex tracking number is already generated when the user marks a particular order as COMPLETE or PAYMENT COMPLETED. </p>