0.2.3: Add config option for "authentication base url" (#18)

* Add "authenticationBaseUrl" to Frontegg Config

Hybrid mode may require different base urls for authentication and API requests. This
is because hybrid mode requires authentication via Frontegg Cloud but the actual API
endpoints will by hosted by the local hybrid instance. So authentication will go through
api.frontegg.com and other API calls will go to whatever url is defined by the hybrid
installation.

* Use authentication url from config to authenticate instead of service url

* Include "authenticationBaseUrl" in readme