Email alerter

[sbzgen]

Successor to #3 which needed a rewrite after all the changes in #56.

Untested in your codebase, but this was backported from a hard fork of mine that was heavily rewritten. I have other improvements that can come later but it'd be better to do them one at a time :)

Mail providers can be finicky with what they support and don't support. My solution uses the most compatible approach which most providers will accept.

My solution was to add a command that lets users test their selected alerter to see if it works without any problems, but I don't know if you would want to add that. I could also catch all smtp exceptions and log them.