Support enable_key_lonlat

Add support `enable_key_lonlat`, to out an array property that includes [`geoip_longitude`, `geoip_lattitude`].

This change is a little bit tricky, but it's very useful to integrate with Kibana's bettermap.
Because to use bettermap, we need an array property that includes [`longitude`, `lattitude`].

Ref: similar issue of logstash
https://github.com/logstash/logstash/pull/582
https://github.com/logstash/logstash/pull/765

I use this feature on our production server for a week, and it works well.