Add more response formats to login endpoint

[scottrobertson]

This should support more than just JSON.

https://github.com/tokenable/tokenable-ruby/blob/main/lib/tokenable/controllers/tokens_controller.rb#L8

Once #18 is done, this should be super easy.

[scottrobertson]

We will need to spec out what happens when they post with HTML content-type, and XML.