Nowadays services.{redshift,gammastep} modules are really similar. They
should, since Gammastep is a fork of Redshift with the main objective is
to support Wayland.
So instead of trying to maintain two separate modules, this commit unify
the options in lib/options.nix file, making the implementation of the
module itself ends up being really simple (just calling the common
options with the necessary parameters to differentiate between them).
