Changing Direction While In Mid-Air?

I'm creating a 2D platformer, and the character control script from the Unity API is working decently well. The only concern I have with it is that you cannot go left or right when you are in mid-jump. How can I make the script do this?

 var speed : float = 6.0;
 var jumpSpeed : float = 8.0;
 var gravity : float = 20.0;
 
 private var moveDirection : Vector3 = Vector3.zero;
 
 function Update() {
     var controller : CharacterController = GetComponent(CharacterController);
     if (controller.isGrounded) {
         // We are grounded, so recalculate
         // move direction directly from axes
         moveDirection = Vector3(Input.GetAxis("Horizontal"), 0,
                                 0);
         moveDirection = transform.TransformDirection(moveDirection);
         moveDirection *= speed;
         
         if (Input.GetButton ("Jump")) {
             moveDirection.y = jumpSpeed;
         }
     }
 
     // Apply gravity
     moveDirection.y -= gravity * Time.deltaTime;
     
     // Move the controller
     controller.Move(moveDirection * Time.deltaTime);
 }