Improved changes made in 9c8799f, allowing to pass kwargs to ES

In 9c8799f (PR #228), smaddineni updated generate_model to handle non-localhost connections.
However it would be better to support keyword-based arguments to define, for example `basic_auth`, `timeout`, etc.
I've kept `es_url` argument for compatibility reasons but it should be deprecated in favor of `es_kwargs`.