Adds support for setting additional client args.

[dylan-stark]

Adds support for setting additional client args to address issue #44.

Essentially, I added a module-level client_args dictionary, similar to huggingface.api_key and .prompt_builder. A user can put headers, cookies, etc., in there. And the key-value pairs will get unpacked at call-time as additional keyword args.

Note that I left the (implicit) model and token args untouched. Perhaps it makes sense to do a little work to check whether those are in client_args ahead of the call.