autojump: creates links required by oh-my-zsh for autojump.

The autojump plugin in oh-my-zsh assumes autojump.zsh resides in
/run/current-system/sw/share/autojump/ but these links are not created
by default.

The new programs.autojump.enable option forces the creation of these
links.

nixos/modules/module-list.nix

@@ -78,6 +78,7 @@
   ./misc/version.nix
   ./programs/adb.nix
   ./programs/atop.nix
+  ./programs/autojump.nix
   ./programs/bash/bash.nix
   ./programs/bcc.nix
   ./programs/blcr.nix

nixos/modules/programs/autojump.nix

@@ -0,0 +1,33 @@
+{ config, lib, pkgs, ... }:
+
+with lib;
+
+let
+
+  cfg = config.programs.autojump;
+
+in
+
+{
+
+  ###### interface
+
+  options = {
+    programs.autojump = {
+
+      enable = mkOption {
+        type = types.bool;
+        default = false;
+        description = ''
+          Whether to enable autojump.
+        '';
+      };
+    };
+  }; 
+
+  ###### implementation
+
+  config = mkIf cfg.enable {
+    environment.pathsToLink = [ "/share/autojump" ];
+  };
+}