Feature Extraction endpoint is not well defined.

[Kardbord]

As of the initial writing of this library, the feature extraction endpoint doesn't seem to be well defined. At the moment it seems like the request structure varies from model to model. That may or may not be the case, and its likely that I just don't understand what's going on, but unfortunately I don't have the free time to expend on figuring out what's going on. I'll wait until the docs become more explicit to implement this endpoint.