add_str() and add() do not take the same optional parameters

[ftyers]

e.g.

				sent_hash = self._client.add_json(sentence, opts=opts)
				clip_res = self._client.add(clip_path, only_hash=True)

See #287

[c0llab0rat0r]

I've pulled @ftyers's changes from #287 into a branch of my fork. Thanks for the submission and we'll continue its work.