Adds an idea for networking module

The idea is to follow: https://nixos.org/nixos/options.html#networking so we can
share even more configuration ideas from NixOS.

default.nix

@@ -31,6 +31,7 @@ let
         ./modules/system/etc.nix
         ./modules/system/launchd.nix
         ./modules/time
+        ./modules/networking
         ./modules/nix
         ./modules/nix/nix-darwin.nix
         ./modules/nix/nixpkgs.nix

modules/networking/default.nix

@@ -0,0 +1,41 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.networking;
+
+  hostName = optionalString (cfg.hostName != null) ''
+    scutil --set ComputerName "${cfg.hostName}"
+    scutil --set LocalHostName "${cfg.hostName}"
+    scutil --set HostName "${cfg.hostName}"
+  '';
+
+in
+
+{
+  options = {
+
+    networking.hostName = mkOption {
+      type = types.nullOr types.str;
+      default = null;
+      example = "myhostname";
+      description = ''
+        Hostname for your machine.
+      '';
+    };
+
+  };
+
+  config = {
+
+    system.activationScripts.networking.text = ''
+      # Set defaults
+      echo "configuring networking..." >&2
+
+      ${hostName}
+    '';
+
+  };
+}

modules/system/activation-scripts.nix

@@ -60,6 +60,7 @@ in
       ${cfg.activationScripts.etc.text}
       ${cfg.activationScripts.launchd.text}
       ${cfg.activationScripts.time.text}
+      ${cfg.activationScripts.networking.text}
 
       # Make this configuration the current configuration.
       # The readlink is there to ensure that when $systemConfig = /system