rxvt_unicode-with-plugins: passthru plugins

Because we hardcode a list of useful plugins, it may be useful for
someone who's overriding this.

See https://github.com/NixOS/nixpkgs/issues/15743#issuecomment-222112433

pkgs/applications/misc/rxvt_unicode/wrapper.nix

@@ -16,4 +16,6 @@ in symlinkJoin {
     wrapProgram $out/bin/urxvtd \
       --suffix-each URXVT_PERL_LIB ':' "$out/lib/urxvt/perl"
   '';
+
+  passthru.plugins = plugins;
 }