Add end-to-end encryption support to SimplePush

Hey there! Big fan of ntfy and have used it for years. Simplepush (https://github.com/simplepush) now supports end-to-end encryption for their Android app notifications, and it's pretty easy to utilize. It'd be awesome if ntfy could support this too.

Here is their public code showinghow encrypted notifications work::
https://github.com/simplepush/simplepush-python/blob/master/src/simplepush/simplepush.py

Or an even easier BASH script:
https://github.com/simplepush/send-encrypted/blob/master/simplepush.sh

Thanks!