npm module: add npm as nixos program

nixos/modules/module-list.nix

@@ -78,6 +78,7 @@
   ./programs/man.nix
   ./programs/mosh.nix
   ./programs/nano.nix
+  ./programs/npm.nix
   ./programs/oblogout.nix
   ./programs/screen.nix
   ./programs/shadow.nix

nixos/modules/programs/npm.nix

@@ -0,0 +1,44 @@
+{ config, lib, ... }:
+
+with lib;
+
+let
+  cfg = config.programs.npm;
+in
+
+{
+  ###### interface
+
+  options = {
+    programs.npm = {
+      enable = mkEnableOption "<command>npm</command> global config";
+
+      npmrc = lib.mkOption {
+        type = lib.types.lines;
+        description = ''
+          The system-wide npm configuration.
+          See <link xlink:href="https://docs.npmjs.com/misc/config"/>.
+        '';
+        default = ''
+          prefix = ''${HOME}/.npm
+        '';
+        example = ''
+          prefix = ''${HOME}/.npm
+          https-proxy=proxy.example.com
+          init-license=MIT
+          init-author-url=http://npmjs.org
+          color=true
+        '';
+      };
+    };
+  };
+
+  ###### implementation
+
+  config = lib.mkIf cfg.enable {
+    environment.etc."npmrc".text = cfg.npmrc;
+
+    environment.variables.NPM_CONFIG_GLOBALCONFIG = "/etc/npmrc";
+  };
+
+}