Add support for apiVersion option in elasticsearch client

[pekastel]

Hi, @jbdemonte

I'm facing an issue while creating documents on older ElasticSearch Servers (5.6)

Error: [invalid_type_name_exception] Document mapping type name can't start with '_', found: [_doc]

An easy fix for this kind of errors would be to add support for apiVersion option on the ElasticSearch client.

I can prepare a pull request if you want.