Convert LogstashAccessFormatter to using Jackson streaming apis for better performance
Added new/flexible way of adding custom event-specific fields to the logstash json event using Markers.

Deprecated old way of using "JSON" marker and using enableContextMap config.

The new way is much more flexible and intuitive.

Updated README.md with lots of documentation on how to use the Markers to add custom event-specific fields to the logstash json event.