Add unload alert

src/demo.js

@@ -6,6 +6,7 @@ require(['config/require-config'], function() {
 
 
 		{
+      noticeOnUnload: 0,
 			container	: '#gjs',
 			height: '100%',
 			fromElement: true,

src/grapesjs/config/config.js

@@ -22,6 +22,9 @@ define(function () {
     // Enable/Disable undo manager
     undoManager: true,
 
+    // Show an alert before unload the page
+    noticeOnUnload: true,
+
     // Storage Manager
     storage: {},
 

src/grapesjs/main.js

@@ -45,6 +45,11 @@ define(function (require) {
         if(!els)
           throw new Error("'container' is required");
 
+        if(c.noticeOnUnload)
+          window.onbeforeunload = function(e) {
+            return 1;
+          };
+
         c.el = document.querySelector(els);
         var editor = new Editor(c).init();