Email notifications

[jdmedlock]

Is your feature request related to a problem? Please describe.
This is not related to a problem. It's purely a new feature request.

Describe the solution you'd like
When a monitored service doesn't respond to Kerner's health check there should be an option for it to email an interested party. For example, the admin for that service. Other notification types would be nice, like SMS or DMing a person on a site like Discord or Slack, but not required for the initial implementation of this feature.

Describe alternatives you've considered
A clear and concise description of any alternative solutions or features you've considered.

Additional context
The email notification interval should be separate from the polling interval for the health check to prevent flooding the interested parties inbox. It may also be useful to implement this as two separate intervals - one interval for the initial detection of the outage and a second interval used to send reminders. For example, 1 minute for the first and increments of 15 minutes for the second.

[jdmedlock]

I know this is in your roadmap for upcoming features. But, I hope this is useful none-the-less.