Add ability to make root row vertical

[slonoed]

Right now the orientation of the root row always horizontal. https://github.com/caplin/FlexLayout/blob/master/src/model/Node.ts#L81

Would be nice to have an option to change it to vertical.

As a workaround I'm using this on consumer side:

    model._root.getOrientation = () => {
      return Orientation.VERT;
    };

This could be part of global configuration.