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's a long shot, but is it possible that "_source" field has a document w/ a @metadata field? It will be then replaced if there's @docinfo.

To prevent this, maybe something like

```ruby
event[@docinfo_target] ||= {}
raise Exception.new("incompatible event") unless event[@docinfo_target].is_a? Hash
```

It's a longshot so it might not be necessary...what do you think?