title: findOneAndUpdate increment instead of update in mongoose
body: <p>I have a function that suppose to add up or decrease credit. But my below code merely replace the credit value. Is there any shortcut in mongoose that I can do increment or decrement? else I will have to get the value then insert it back, which I think is not the way to do it.</p>
  
  <pre><code>function update_total_credit(total_amount, topup_value){
              User.findOneAndUpdate(
                  {email: user_email}, 
                  {$set:{credit:total_amount}}, 
                  {new: true}, 
                  function(err, response){
                  if(err){
                      res.json(0);
                  }else{
                      res.json(response.credit);
                  }
              });
  
          }
  </code></pre>