Provide opt-in email for notification on failed build

[bollwyvl]

As a maintainer, when my kernel fails some tests, I would like to get an email!

In kernel/xyzkernel/meta.whatever, offer a place to capture notifications, starting with email:

"notifications": {
  "email": ["maintainer@example.com", "mailing-list@project.org"],
  "github": "user/project"
}

Investigate proper way to do sending/remailing for notifications.

[takluyver]

@rgbkrk knows something about sending out emails, I think.

[rgbkrk]

"Use mailgun, feel happy" is my primary advice.

[takluyver]

Thanks! And it appears to be free up to 10k emails per month, which should be plenty for this.