ItemComponent should have classname for isOpen

[fczuardi]

Following the examples on the README doc, one possible way to apply custom styles on a tree menu is to use the ItemComponent component.

The generated html <li> tags have classNames for indicating active and focused states (rstm-tree-item--active and rstm-tree-item--focused) but dont include one for the isOpen state. Here.

It would be nice to have one extra className for making custom style for open tree items easier via css.