Add typings for onDragUpdate

While mentioned in the docs, attempting to use onDragUpdate will cause a type error. This PR introduces typings for onDragUpdate that are consistent with the docs:

`type DragUpdate = {|
  ...DragStart,
  // may not have any destination (drag to nowhere)
  destination: ?DraggableLocation,
|}`

types/react-beautiful-dnd/index.d.ts

@@ -28,6 +28,10 @@ export interface DragStart {
     source: DraggableLocation;
 }
 
+export interface DragUpdate extends DragStart {
+    destination?: DraggableLocation;
+}
+
 export interface DropResult {
     draggableId: DraggableId;
     type: TypeId;
@@ -37,6 +41,7 @@ export interface DropResult {
 
 export interface DragDropContextProps {
     onDragStart?(initial: DragStart): void;
+    onDragUpdate?(initial: DragUpdate): void;
     onDragEnd(result: DropResult): void;
 }