Replace workflow's `:config-auth` with a `:credential-fn` that accepts the OAuth2 token and (if authorized) returns a credential map.

This allows middleware consumers to use the token as an identity, which isn't exactly in the spirit of OAuth2 but is how everyone uses it.
See #10 for discussion.
Merge pull request #11 from lynaghk/0.1.0-pre

Replace workflow's `:config-auth` with a `:credential-fn`