options version 3x

As far as i understand the option should be changed according https://github.com/jakubpawlowicz/clean-css#how-to-upgrade-clean-css-from-2x-to-3x

Both `advanced` and `rebase` are set to false by default now.
Additionally since now  rebase  is  off  by default the  --skip-rebase  option does not make sense anymore and instead the plugin needs option to turn it  on  (i.e. just  --rebase ?)