[OverlayMenu] allow menuClassName prop

src/overlayMenu/OverlayMenu.jsx

@@ -1,12 +1,13 @@
 import cx from 'classnames';
 import style from './overlayMenu.css';
 
-function OverlayMenu ({renderMenu, className, ...props}, children) {
+function OverlayMenu ({renderMenu, className, menuClassName, ...props}, children) {
     const composedClassName = cx(className, style.overlayMenu);
+    const composedMenuClassName = cx(menuClassName, style.menu);
 
     return (
         <div className={composedClassName} {...props}>
-            <div className={style.menu}>
+            <div className={composedMenuClassName}>
                 {renderMenu()}
             </div>