root option: add home-manager.sharedModules

This option is a list of modules shared among all users to avoid
importing the same modules for every user manually.

nixos/default.nix

@@ -34,6 +34,7 @@ let
           home.homeDirectory = config.users.users.${name}.home;
         };
       })
+      ({ ... }: { imports = cfg.sharedModules; })
     ];
   };
 
@@ -65,6 +66,15 @@ in {
         '';
       };
 
+      sharedModules = mkOption {
+        type = types.listOf types.path;
+        default = [ ];
+        example = [ /path/to/module.nix ];
+        description = ''
+          Shared home-manager modules among all users
+        '';
+      };
+
       verbose = mkEnableOption "verbose output on activation";
 
       users = mkOption {