feat(Controller): add method for getting trigger axis value

There is a new method on the Controller Events for getting the
current position of the trigger axis rather than needing to
subscribe to the event which may not always be desireable.

Assets/SteamVR_Unity_Toolkit/Scripts/VRTK_ControllerEvents.cs

@@ -297,6 +297,11 @@ public float GetTouchpadAxisAngle()
             return CalculateTouchpadAxisAngle(touchpadAxis);
         }
 
+        public float GetTriggerAxis()
+        {
+            return triggerAxis.x;
+        }
+
         public bool AnyButtonPressed()
         {
             return (triggerPressed || gripPressed || touchpadPressed || applicationMenuPressed);