services.fluentd: add plugins option

This allows us to pass in additional ad-hoc fluentd plugins for custom
output formats and other goodness.

nixos/modules/services/logging/fluentd.nix

···
 
 let
   cfg = config.services.fluentd;
+
+  pluginArgs = concatStringsSep " " (map (x: "-p ${x}") cfg.plugins);
 in {
   ###### interface
 
···
         defaultText = "pkgs.fluentd";
         description = "The fluentd package to use.";
       };
+
+      plugins = mkOption {
+        type = types.listOf types.path;
+        default = [];
+        description = ''
+          A list of plugin paths to pass into fluentd. It will make plugins defined in ruby files
+          there available in your config.
+        '';
+      };
     };
   };
 
···
       description = "Fluentd Daemon";
       wantedBy = [ "multi-user.target" ];
       serviceConfig = {
-        ExecStart = "${cfg.package}/bin/fluentd -c ${pkgs.writeText "fluentd.conf" cfg.config}";
+        ExecStart = "${cfg.package}/bin/fluentd -c ${pkgs.writeText "fluentd.conf" cfg.config} ${pluginArgs}";
         ExecReload = "${pkgs.coreutils}/bin/kill -HUP $MAINPID";
       };
     };