Add ntfy.sh as a backend

[binwiederhier]

Hello hello,

turns out I made a project that has the same name as yours and integrates well with your project. Someone on Reddit suggested that I integrate the two projects, so here I am. It was really simple and it works wonderfully. Great project.

Here's an example usage:

$ ntfy -b ntfysh -o topic mytopic send Howdy

Let me know if there is anything else I should provide or update (Readme, docs, ...)