Write all item fields as data-? attributes on element

If the dataAttributes config option = 'all' then write every field as
data-? attributes on the element.

lib/timeline/component/item/Item.js

@@ -201,8 +201,20 @@ Item.prototype._updateTitle = function (element) {
  */
  Item.prototype._updateDataAttributes = function(element) {
   if (this.options.dataAttributes && this.options.dataAttributes.length > 0) {
-    for (var i = 0; i < this.options.dataAttributes.length; i++) {
-      var name = this.options.dataAttributes[i];
+    var attributes = [];
+
+    if (Array.isArray(this.options.dataAttributes)) {
+      attributes = this.options.dataAttributes;
+    }
+    else if (this.options.dataAttributes == 'all') {
+      attributes = Object.keys(this.data);
+    }
+    else {
+      return;
+    }
+
+    for (var i = 0; i < attributes.length; i++) {
+      var name = attributes[i];
       var value = this.data[name];
 
       if (value != null) {