add slack notification cli flag

[ubergesundheit]

This pull request adds sending of notifications to slack channels.

Please review the glide.yaml and glide.lock files

[rdamazio]

This app already uses logrus which can be hooked to slack - why not just use that? (unfortunately that doesn't work for email, since logrus email integration only sends errors)

[Nightbr]

Hey, the idea is pretty cool, I would really enjoy slack notification for watchtower.
If you still want to contribute on this feature, look at amazing work of @rdamazio here #106 and you can try to implement Slack notification with logrus and this integration https://github.com/johntdyer/slackrus

[stffabi]

Yeah, would be great to have this included in the same release as the email notifications.

[ubergesundheit]

I'll see if I am able to change everything to use logrus/slackrus

[ubergesundheit]

See #113