Add support for additional notification methods

[lrstanley]

Options:

• https://github.com/containrrr/shoutrrr (easier to configure, just allow users to pass in url's via cli or env)
• https://github.com/nikoksr/notify

This would likely be a breaking change for existing email support.

[zamazan4ik]

@lrstanley Could you please specify somewhere in the documentation (in the README file, I think) the supported notification mechanisms right now? For now, it's unclear which exact notification ways could be used.

As far as I understand, mail notification is the only supported mechanism, am I right?

[lrstanley]

@zamazan4ik email is the only supported option, that is correct. I will be taking a look at implementing more options (and deprecating how mail notifications currently work) very soon, so I will likely wait until I have the new solution in place before updating the readme.

[c33s]

@lrstanley a webhook notification and/or a shell script notification would be best, having a webhook practically allows to do nearly everything. if it is to complicated to implement a full webhook support (headers, POST, GET), simply calling a custom shell script would be enough. having a shellscript as handler allows admins to really allow every form of notification.