Add support for AstroNote

[st3fan]

This patch adds support for AstroNote, which is a small push notifications app for Apple Watch.

I've marked the PR as Draft - I think it is complete but I'd love to hear if @dschep is interested in including this backend and if so, if anything is missing in this patch.

[coveralls]

Coverage increased (+0.5%) to 81.623% when pulling 50f1853 on st3fan:st3fan/astronote into ecfeee9 on dschep:master.

[dschep]

Hey @st3fan, this looks good for the most part. However, some time ago, I added the capability for ntfy to load backends from 3rd party modules, my official recomendation is that you publish this as It's own package, such as ntfy-astronote, that way updates to the astronote backend aren't blocked by me making new releases of ntfy. You can check out my ntfy-webpush library for an example of this: https://github.com/dschep/ntfy-webpush

[dschep]

note, for discoverability, I'll still gladly accept a PR adding the new backend to the list of backends just as I've added for webpush once you've published your own package