Add a `pull_request:merged` event

[patcon]

Subset of #9

payload for closed PR: https://gist.github.com/patcon/11215810#file-closed-pull-request-json

{
//...
  "merged": false,
  "mergeable": true,
  "mergeable_state": "clean",
  "merged_by": null,
/...
}

payload for a merged PR: https://gist.github.com/patcon/11215810#file-merged-pull-request-json

{
//...
  "merged": true,
  "mergeable": null,
  "mergeable_state": "unknown",
  "merged_by": {
    "login": "whit537",
    "id": 134455,
    "avatar_url": "https://avatars.githubusercontent.com/u/134455?",
    "gravatar_id": "fb054b407a6461e417ee6b6ae084da37",
//...
}

[strugee]

Fixed in e61648aa