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
- Change the default query to elasticsearch to  '{"query": { "match_all": {} } }' 

I took this https://github.com/elasticsearch/logstash/pull/1644  as the original starting point 
It might be important to note, with regards to metadata, that if you're ingesting documents with the intent to re-index them (or just update them) that the `action` option in the elasticsearch output want's to know how to handle those things.  It can be dynamically assigned with a field added to the metadata.

I'm not sure how much of this _needs_ to be in here, but I thought it worth mentioning.