config_param log_level (v11)

What I want to do with this patch is to configure log_level in each plugin. To do it, I newly created a plugin option, `log_level` which is broadly available by *all* plugins. I created a top-level class of all plugins named `Fluentd::Plugin::Agent` to introduce the broad option. 

HOW TO USE: An example of `fluentd.rb` is as followings:

    worker {
      match('raw.syslog') {
        type :stdout
        log_level "error"
      }
    }

Inside of the plugin code, plugin creators should write as

    logger.info "foobar"