added ReactElement type for title prop in MenuItemGroupProps (#294)

* added ReactElement as supported type for title prop in MenuItemGroupProps interface

* switch to ReactNode instead of ReactElement

* remove type string as it is included in ReactNode

src/MenuItemGroup.tsx

@@ -13,7 +13,7 @@ export interface MenuItemGroupProps {
   className?: string;
   subMenuKey?: string;
   rootPrefixCls?: string;
-  title?: string;
+  title?: React.ReactNode;
   onClick?: MenuClickEventHandler;
 }