Feature/allow the use of metadata

- Allow to store Elasticsearch document information in the `@metadata` field or somewhere else.
- Allow the user to specify which fields to move,  the default is _index, _type, _id

I took this https://github.com/elasticsearch/logstash/pull/1644  as the original starting point 
Reading the older code also - I'm confused how this works at all, hah

`event` here is a Hash, not a LogStash::Event - so something weird is happening. Let's add tests to cover the object type being shipped to the output_queue.