Support detection of project ID

[stephenplusplus]

RE: googleapis/google-cloud-node#1653

It would be nice if the authClient returned from this module also exported its best guess at the project_id. In the case of service account JSON files, they have the project_id set, so exporting that should be simple. For ADC, it would involve spawning a command:

$ gcloud -q config list core/project --format=json
{
  "core": {
    "project": "grape-spaceship-123"
  }
}

cc @tmatsuo @jmdobry