Major config overhaul: use custom modules, setup for multi-host config, and less boilerplate

modules/system/printing/default.nix

@@ -0,0 +1,20 @@
+{ pkgs, lib, config, ... }:
+
+let
+  cfg = config.systemSettings.printing;
+in {
+  options = {
+    systemSettings.printing = {
+      enable = lib.mkEnableOption "Enable printing";
+    };
+  };
+
+  config = lib.mkIf cfg.enable {
+    # Enable printing
+    services.printing.enable = true;
+    services.avahi.enable = true;
+    services.avahi.nssmdns4 = true;
+    services.avahi.openFirewall = true;
+    environment.systemPackages = [ pkgs.cups-filters ];
+  };
+}