Add is-open class to the button when dropdown is open

Enables styling for the button specific to the open state, such as rotating the arrow.

src/javascript/vanilla-js-dropdown.js

@@ -108,7 +108,7 @@ var CustomSelect = function(options) {
 
     var t = e.target; // || e.srcElement; - uncomment for IE8
 
-    if (t.className === titleClass) {
+    if (t.className.includes(titleClass)) {
       toggle();
     }
 
@@ -137,6 +137,7 @@ var CustomSelect = function(options) {
    */
   function toggle() {
     ul.classList.toggle(openClass);
+    button.classList.toggle(openClass);
   }
 
   /**
@@ -146,6 +147,7 @@ var CustomSelect = function(options) {
    */
   function open() {
     ul.classList.add(openClass);
+    button.classList.add(openClass);
   }
 
   /**
@@ -155,6 +157,7 @@ var CustomSelect = function(options) {
    */
   function close() {
     ul.classList.remove(openClass);
+    button.classList.remove(openClass);
   }
 
   return {