Support :transit-params

Allow `:transit-params` key that sets content type and encodes as transit -- decodes response body with content type `application/transit+json`.
I think I've attempted to answer this three comments above -- the `default-transit-opts` defined the the private Var will be used in absence of `:transit-opts` in the request, making the passing of encoding and decoding options optional.

If you're meaning passing transit-opts in the arglist, I think I'm improperly using this library.