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 agree this should be moved to the helpers module.
I will refactor all the test in the spec to use the same method.
I will keep this method inside this spec for now and open a PR on devutils.