implement options.levelVisible, https://github.com/phetsims/vegas/issues/71
implement options.levelVisible, https://github.com/phetsims/vegas/issues/71

Signed-off-by: Chris Malley <74b2712074b149d140079dbd7e875955d192e617@pixelzoom.com>

phetsims/vegas,phetsims/vegas,phetsims/vegas