Add support of extra styling class on collapse event #11165

User has the possibility to add a user-defined 'class' of his own preference that can be used for formatting a panel when it is collapsed in. For enabling this functionality user can simply add **data-class="any-class"** attribute on the button/link toggle.

You can check the result [at this JS Bin](http://jsbin.com/IsOXIWe/4/).