Add support for multiple audiences with OIDC

[danschultzer]

Resolves #111.

It's required to set a list of trusted audiences in the config:

config =  [
  client_id: "REPLACE_WITH_CLIENT_ID",
  site: "https://server.example.com",
  authorization_params: [scope: "user:read user:write"],
  trusted_audiences: ~w(aud1 aud2)
]