Add Dead Man's Snitch as a notifier.

[Dead Man's Snitch](https://deadmanssnitch.com/) is a service I create that lets you know when something doesn't happen during a specified interval. This is the code to add DMS as a notifier.

I override all of the on_* configs since the service really wants to know about anything but failure. The one question I had was, if there is a warning, does that still count as success? Maybe I should leave the on_warning up to the user but override the on_success and on_failure?