title: Woocommerce - update cart button, custom function
body: <p>Im in need to create custom update button function in the cart.
  Basically I need to add additional input (next to quantity) in the cart and depending on the input value i need to change price of the product in the cart.
  Where can I modify function which runs after I hit the update cart button? PHP function together with JS function? Any idea where are those files located and what are function names? 
  Thanks in adance</p>