fix: make DraggableCore bounds optional in typings

Author: Grady Hallenbeck

typings/index.d.ts

@@ -2,10 +2,10 @@ declare module 'react-draggable' {
   import * as React from 'react';
 
   export interface DraggableBounds {
-    left: number
-    right: number
-    top: number
-    bottom: number
+    left?: number
+    right?: number
+    top?: number
+    bottom?: number
   }
 
   export interface DraggableProps extends DraggableCoreProps {