Make user_token and user_email configurable

Rather than forcing the API to return a response with `user_token` and `user_email` in order for the session to be persisted, you should be able to configure what properties to look for in the response. This adds `tokenAttributeName` and `emailAttributeName` that may be configured to reflect the API that Simple Auth is interacting with. Our API for instance returns a new session response like:

```json
{
  "user": {
    "token": "someToken",
    "email": "hello@example.com"
  }
}
```

This would allow us to configure the `tokenAttributeName` to be `'user.token'` and the `emailAttributeName` to be `'user.email'`.