out: use compat_parameters helper for v0.14

It keeps compatibility with v0.12 style configuration.

ref. http://docs.fluentd.org/v0.14/articles/plugin-update-from-v12#buffered-output-plugins

lib/fluent/plugin/out_groonga.rb

@@ -27,6 +27,7 @@ module Fluent
   module Plugin
   class GroongaOutput < Output
     Plugin.register_output("groonga", self)
+    helpers :compat_parameters
 
     def initialize
       super
@@ -73,7 +74,13 @@ def initialize
       end
     end
 
+    config_section :buffer do
+      config_set_default :@type, "memory"
+      config_set_default :chunk_keys, ['tag']
+    end
+
     def configure(conf)
+      compat_parameters_convert(conf, :buffer)
       super
       @client = create_client(@protocol)
       @client.configure(conf)