Webhook mocks/fixtures

[gr2m]

moving from gr2m/octokit-webhooks.js-legacy#8 @rtsao, @JasonEtco

---

Per probot/probot#316 (comment), for testing probot apps, it would be ideal to have a node-github-like API to programmatically generate webhook fixtures, for example something like:

let robot = /* ... */

let github = new MockGitHub();
let repo = github.mocks.repo({
  org: 'test-org',
  name: 'test-repo'
});

robot.auth = () => Promise.resolve(github);

let webhook = github.issues.edited(
  repo.issue({
    body: 'new issue body'
  })
);

await robot.recieve(webhook);

/* ... assertions ... */