Add ntfy appender

[jonocarroll]

{ntfy} (CRAN) is similar to {RPushbullet} but requires no sign-up, registration, or subscription - it's a pub/sub service, and a good fit for an appender, I think.

{ntfy} is currently also available as a {progressr} handler.

(prompted by your useR! talk which has just been uploaded - great work!)

[daroczig]

This is really cool, thank you!

I've left a minor comment, please review. Also, it would be great to update NEWS.md and document title and tags to fix the failing linter.

[daroczig]

thanks! all looks good, except the linter failing due to some trailing whitespace 🤐

[jonocarroll]

In theory the air formatter solves these problems, but it's a big change the first time you run it so I have to keep turning it on/off for different projects. Then I forget to run linters.