Make authentication config a bit more forgiving

Similarly to the other request, this plugin is currently pretty unforgiving with auth details: if you provide a 'username' rather than an 'email', it will ask you for a password, and then happily tell you that login failed, rather than pointing out you didn't really provide enough details.

With this patch, correct configuration is now validated up front, and some extra useful notes on this have been added to the readme to clarify ambiguity about which auth details can be used.