To correspond to the _routing field of Elasticsearch 6.x

[chobitsky]

Hi,

From Elasticsearch 6.x you have to specify the routing field when parent and child data is populated.
Reference link: https://www.elastic.co/guide/en/elasticsearch/reference/current/mapping-routing-field.html

Currently, as far as the source code of the embulk-output-elasticsearch plugin is concerned,
there seems to be no _routing specification, so I would like to add this.

[sakama]

Sorry for late response. I overlooked this issue.

Although I'm not aware about routing feature, to support this feature sounds good to me.
We welcome your PR.

Thanks.