I know I can set ion auth to login by username in the config, I also know I can set it to login by email in the config.
Is there a simple way to set it it to use either automatically?
If by automatic, you mean try one and then the other to see if either gives a valid return:
The login occurs in ion_auth_model line: 899
so you could change this to do an "or" and try both columns. You would need to do this throughout the model because there is more than just the actual login to consider & there's the potential issue of a user having an email that is another user's username (however unlikely)