Add addLabel() to add a label to an issue

Author: asmsuechan

lib/Issue.js

@@ -246,6 +246,18 @@ class Issue extends Requestable {
    deleteLabel(label, cb) {
       return this._request('DELETE', `/repos/${this.__repository}/labels/${label}`, null, cb);
    }
+
+  /**
+   * Add label to an issue
+   * @see https://developer.github.com/v3/issues/labels/#add-labels-to-an-issue
+   * @param {number} issue - the id of the issue to comment on
+   * @param {string} label - the name of the label to add to the issue
+   * @param {Requestable.callback} [cb] - will receive the status
+   * @return {Promise} - the promise for the http request
+   */
+   addLabel(issue, label, cb) {
+      return this._request('POST', `/repos/${this.__repository}/issues/${issue}/labels`, label, cb);
+   }
 }
 
 module.exports = Issue;