Support ntfy.sh

[haxwithaxe]

I would like support for https://nyfy.sh as a provider. It's the only thing preventing me from using your library. ntfy.sh is a bit like the email provider in that the base_url changes when it's selfhosted. Otherwise it has a json API so it's simple to send the broad range of fields without having to worrying about much.

I am interested in providing a pull request if you are interested in adding support.