Attach forceValidate event to attributes

Allow attributes to have their validation triggered by a custom event.
`validateAttribute` is called with `forceValidate` set to true.

framework/assets/yii.activeForm.js

@@ -413,6 +413,9 @@
                 }
             });
         }
+        $input.on('forceValidate.yiiActiveForm', function() {
+            validateAttribute($form, attribute, true);
+        });
     };
 
     var unwatchAttribute = function ($form, attribute) {