Output plugins MUST have actors
 * include Actor::AgentMixin on Output to provide #actor method for all output plugins
 * call 'super' on BufferedOutput #start/#stop/#shutdown to initiate actors
 * add 2 plugins to check features of Fluentd itself
 * add 'example' directory to save configuration files to
   * check configurations to check Fluentd itself
   * show how to write configurations