generic OAuth

[Paul Poputa-Clean]

Hey,

Noticed the code for generic oauth in github, but no examples for the configs (https://github.com/jupyterhub/oauthenticator/blob/master/oauthenticator/generic.py).

Trying to set it up with PingFed but haven't seen many details outside of github / google / gitlabs (which sound less generic oauth and more purpose built).