Added onbeforechange callback

Author: drgomesp

intro.js

@@ -159,6 +159,10 @@
    * @method _nextStep
    */
   function _nextStep() {
+    if (typeof (this._introBeforeChangeCallback) !== 'undefined') {
+      this._introBeforeChangeCallback.call(this, this._targetElement);
+    }
+
     if (typeof (this._currentStep) === 'undefined') {
       this._currentStep = 0;
     } else {
@@ -189,6 +193,10 @@
       return false;
     }
 
+    if (typeof (this._introBeforeChangeCallback) !== 'undefined') {
+      this._introBeforeChangeCallback.call(this, this._targetElement);
+    }
+
     _showElement.call(this, this._introItems[--this._currentStep]);
   }
 
@@ -702,6 +710,14 @@
     exit: function() {
       _exitIntro.call(this, this._targetElement);
     },
+    onbeforechange: function(providedCallback) {
+      if (typeof (providedCallback) === 'function') {
+        this._introBeforeChangeCallback = providedCallback;
+      } else {
+        throw new Error('Provided callback for onbeforechange was not a function');
+      }
+      return this;
+    },
     onchange: function(providedCallback) {
       if (typeof (providedCallback) === 'function') {
         this._introChangeCallback = providedCallback;