programs/bash: install nix-bash-completions if completion is enabled

nixos/doc/manual/release-notes/rl-1803.xml

···
       and <literal>stopJob</literal> provide an optional <literal>$user</literal> argument for that purpose.
     </para>
   </listitem>
+  <listitem>
+    <para>
+      Enabling bash completion on NixOS, <literal>programs.bash.enableCompletion</literal>, will now also enable
+      completion for the Nix command line tools by installing the
+      <link xlink:href="https://github.com/hedning/nix-bash-completions">nix-bash-completions</link> package.
+    </para>
+  </listitem>
 </itemizedlist>
 
 </section>

nixos/modules/programs/bash/bash.nix

···
       "/share/bash-completion"
     ];
 
+    environment.systemPackages = optional cfg.enableCompletion
+      pkgs.nix-bash-completions;
+
     environment.shells =
       [ "/run/current-system/sw/bin/bash"
         "/var/run/current-system/sw/bin/bash"