payload: add Label to PullRequestPayload

[samuelkarp]

Label now appears to be part of the pull request payload when the action is labeled, with a structure like this:

  "label": {
    "id": 123456789,
    "url": "https://api.github.com/repos/samuelkarp/my-repo/labels/my-label",
    "name": "my-label",
    "color": "e6e6e6",
    "default": false
  },

[deankarn]

Thanks @samuelkarp will review as soon as I get a chance :)