Whitelist Google users based on their email address instead of the username part of the email address.
Plain gmail (non Google Apps) accounts don't necessarily end in '@gmail.com', and whitelisting on username alone is open to abuse.
Skip testing for a Google Apps domain (ui.Hd) if no domain is configured.
Principals will still be added as the user part of the email address.

For the Github provider, skip checking that the user is a member of an organization is none is configured.

This allows the signing key to be read directly from S3 using a path like
/s3/<bucket>/<path/to/signing.key> or /gcs/<bucket>/<path/to/signing.key>.

Rename 'GoogleOpts' to 'ProviderOpts'.
Rename Principals to AdditionalPrincipals to match the config option.

I can never remember how markdown links work.

Include some notes on configuration