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 
I feel like this whole loop (in the old code) is not right. I think it should be something like:

```diff
+      r['hits']['hits'].each do |result|
+         event = LogStash::Event.new(result["_source"])
+
+         if @docinfo
+           @docinfo_fields.each do |field|
+             event["????[#{field}]" = result[field]
+           end
+         end
```

Where '????' is the @docinfo_target thing, but we have to be aware of `[field][name]` vs `fieldname`