Add support for groups

[Kranzes]

it would be very helpful if you could great a group of hosts that can then be deployed at once.

Nix code example:

nixinate = {
  group.production.hosts = [ "hetznix" "swordfish" ];
  group.production.buildOn = "remote"; # available options are "remote", "local" or a specific host
};

[mrVanDalo]

I use parallel to run deployments in parallel

parallel --tag nix run ".#apps.nixinate.{1}" ::: machine1 machine2 machine3

and created aliases like alias deploy-prodction= ...