[WIP] config_param log_level (v11)

What I want to do with this patch is change log_level in each plugin. To do it, I newly created a plugin option, `log_level` which is broadly available by all plugins. 

An example of `fluentd.rb` is as followings:

    worker {
      match('raw.syslog') {
        type :stdout
        log_level "error"
      }
    }

I created a top-level class of all plugins named `Fluentd::Plugin::Agent` to introduce the common option on all plugins.  

Wondering

1. The name of the top-level plugin class `Fluentd::Plugin::Agent`.