enable to use nested ip in record

I want to use nested key in record for lookup key.

configure bellow:

```
      geoip_lookup_key  host.ip
      enable_key_city   geoip_city
      remove_tag_prefix input.
      add_tag_prefix    geoip.
```

fluentd with this configuration can add geographical information to `{'host' => {'ip' => '66.102.3.80'}, 'message' => 'valid ip'}`.