Generic OAuth userinfo properies now configurable

When connecting to a generic OAuth provider, you are never sure what object you'll be receiving from the userinfo endpoint (it isn't actually specified anywhere). So this commit adds mapping values to the generic oauth configuration section of config.ini, allowing the user to specify which keys in the remote endpoint it expects to read the UserID, Username, DisplayName, and Email from. Default values if unspecified remain as they were before this commit.