sagews: implement output hiding - https://github.com/sagemathinc/smc/issues/762
sagews: implement output hiding - https://github.com/sagemathinc/smc/issues/762

sagemathinc/smc,DrXyzzy/smc,tscholl2/smc,tscholl2/smc,DrXyzzy/smc,tscholl2/smc,DrXyzzy/smc,DrXyzzy/smc,sagemathinc/smc,tscholl2/smc,tscholl2/smc,sagemathinc/smc,sagemathinc/smc