fix smoothscroll not working with shadow DOM (#133)

* fix smoothscroll not working with shadow DOM

Adding the el.host will allow this plugin to work with shadow DOM. Before that, el.parentNode on a shadow-root returned null and failed.

* Update smoothscroll.js

* Update smoothscroll.js

Author: Jbernardyoobic

src/smoothscroll.js

@@ -157,7 +157,7 @@ function polyfill() {
    */
   function findScrollableParent(el) {
     while (el !== d.body && isScrollable(el) === false) {
-      el = el.parentNode;
+      el = el.parentNode || el.host;
     }
 
     return el;