Add support for notification service plugins

[Chronial]

The PRs show that there is a clear demand for other notification services beside email. We have Telegram, Slack, Pushbullet, Apprise and Discord. I do not want to maintain those and consider them out of scope for snapraid-runner. It might be a good idea to add a simple, configurable hook-in point with well-defined API so others can easily provide plugins for their favorite notification service.

[fmoledina]

I haven't looked into it too deeply, but I believe Apprise is a single configurable hook-in point, perhaps with a well-defined API for different notification services.

[cfelicio]

If you can integrate apprise, it will take care of pretty much of notification needs: https://github.com/caronc/apprise

Example of a tool using it: https://github.com/dgtlmoon/changedetection.io

[superjkl]

So I've whipped up a quick and dirty fork with apprise support if there's still any interest https://github.com/superjkl/snapraid-runner