code: function tagIssue (client, repo, number, label) {
    debug("tagIssue", repo, number, label);
  
    client.getIssue(_.merge(extractRepo(repo), {
      "number": number
    })).then(function (issue) {
      var labels = _.pluck(issue.labels, "name");
      if (!_.contains(labels, label)) {
        return client.updateIssue(_.merge(extractRepo(repo), {
          "number": number,
          "labels": labels.concat([label])
        })).then(fromGithubIssue);
      } else {
        return fromGithubIssue(issue);
      }
    });
  }
docs: Add a label (append, not replace) String, Number, String ~→ Issue