Config file should contain provider dictionary

[tarickb]

Suggestion from @valinet in #10:

Specify something in the token file like: provider: "google", or provider: "yahoo", and then, in sasl-xoauth2.conf have something like:

  "providers": [
    {
      "provider": "google",
      "client_id": "id_google",
      "client_secret": "secret_google"
    },
    {
      "provider": "yahoo",
      "client_id": "id_yahoo",
      "client_secret": "secret_yahoo"
    }
  ],
  "log_full_trace_on_failure": "yes"