How do I make character's WASD controls move relative to my orbital camera axis?

Character:

using System.Collections; using System.Collections.Generic; using UnityEngine;

public class testscript : MonoBehaviour {

 public float speed = 6.0F;
 public float jumpSpeed = 8.0F;
 public float gravity = 20.0F;
 private Vector3 moveDirection = Vector3.zero;

 private CharacterController controller;

 private void Awake()
 {
     controller = GetComponent<CharacterController>();
 }

 void Update()
 {
     if (controller.isGrounded)
     {
         moveDirection = new Vector3(Input.GetAxis("Horizontal"), 0, Input.GetAxis("Vertical"));

         Quaternion rotation = Quaternion.LookRotation(moveDirection, Vector3.up);
         if (moveDirection != Vector3.zero)
             transform.rotation = rotation;

         moveDirection *= speed;
         if (Input.GetButton("Jump"))
         {
             moveDirection.y = jumpSpeed;
         }

         Debug.DrawLine(transform.position, moveDirection);

     }
     moveDirection.y -= gravity * Time.deltaTime;
     controller.Move(moveDirection * Time.deltaTime);
 }

}

Camera:

[AddComponentMenu("Camera-Control/Mouse Orbit with zoom")] public class MouseOrbitImproved : MonoBehaviour {

 public Transform target;
 public float distance = 5.0f;
 public float xSpeed = 120.0f;
 public float ySpeed = 120.0f;

 public float yMinLimit = -20f;
 public float yMaxLimit = 80f;

 public float distanceMin = .5f;
 public float distanceMax = 15f;

 private Rigidbody rigidbody;

 float x = 0.0f;
 float y = 0.0f;

 // Use this for initialization
 void Start () 
 {
     Vector3 angles = transform.eulerAngles;
     x = angles.y;
     y = angles.x;

     rigidbody = GetComponent<Rigidbody>();

     // Make the rigid body not change rotation
     if (rigidbody != null)
     {
         rigidbody.freezeRotation = true;
     }
 }

 void LateUpdate () 
 {
     if (target) 
     {
         x += Input.GetAxis("Mouse X") * xSpeed * distance * 0.02f;
         y -= Input.GetAxis("Mouse Y") * ySpeed * 0.02f;

         y = ClampAngle(y, yMinLimit, yMaxLimit);

         Quaternion rotation = Quaternion.Euler(y, x, 0);

         distance = Mathf.Clamp(distance - Input.GetAxis("Mouse ScrollWheel")*5, distanceMin, distanceMax);

         RaycastHit hit;
         if (Physics.Linecast (target.position, transform.position, out hit)) 
         {
             distance -=  hit.distance;
         }
         Vector3 negDistance = new Vector3(0.0f, 0.0f, -distance);
         Vector3 position = rotation * negDistance + target.position;

         transform.rotation = rotation;
         transform.position = position;
     }
 }

 public static float ClampAngle(float angle, float min, float max)
 {
     if (angle < -360F)
         angle += 360F;
     if (angle > 360F)
         angle -= 360F;
     return Mathf.Clamp(angle, min, max);
 }

}