Support email notifications to comitter

[kyrofa]

A feature I've always used in Travis is being notified via email when a build finished. One has the option to never get notifications, get notifications if the build failed, or get notifications always. Such a feature would be very useful in snappy jenkins as well. I think the most important one is upon failure, but ideally I'd be able to get them upon success as well (e.g. so I don't need to babysit commits that are just waiting for tests to pass).

[fgimenez]

Hey @kyrofa thanks for reporting this, it's a feature we definitely need. Until it's ready I've created and added you to the #snapcraft-ci tg channel, currently only bare bones notifications of jobs finished and their status are received there. Stay tuned :)

[kyrofa]

@fgimenez I love it! Thanks for the tg channel!