Inner DSL for fluent.conf

Just a proof of concept.
Not ready for merge.

If name of config file specified using `-c` option is ended with `.rb`, then it is loaded as Config DSL.

Below is example config written using DSL.

```ruby
## built-in TCP input
## $ echo <json> | fluent-cat <tag>
source {
  type :forward
}

# HTTP input
# http://localhost:8888/<tag>?json=<json>
source {
  type :http
  port 8888
}

# Listen DRb for debug
source {
  type :debug_agent
  port 24230
}

## match tag=debug.** and dump to console
match('debug.**') {
  type :stdout
}

# match tag=myapp.** and forward and write to file
match('myapp.**') {
  type :copy
  store {
    type :forward
    host '192.168.0.13'
    buffer_type :file
    buffer_path '/var/log/fluent/myapp-forward'
    retry_limit 50
    flush_interval '10s'
  }
  store {
    type :file
    path '/var/log/fluent/myapp'
  }
}
```

Comments are welcome.