Output log events to match default settings on Kibana logstash dashboard

Hi

Thanks for creating this useful library. I was wondering if you could add support to enable us to use the Kibana Logstash Dashboard without modifications with the logs created by this appender. 

See the diff attached. 

Specifically, the timestamp field is named @timestamp, and the date format on the indiex name is yyyy.MM.dd, not yyyy-MM-dd. If it could be configurable, that is fine of course.