Alert Notifications: Add support for nfty (https://github.com/binwiederhier/ntfy)

[cnupy]

ntfy is an open source simple HTTP-based pub-sub notification service, that lets you send notifications to your phone or desktop via API or CLI. It can be self-hosted and it has an open source Android and iOS applications available on Google Play, F-Droid, and App Store. It also has support for UnifiedPush, an open standard for receiving push notifications, which is also great to have support for as there are other services that support it and can be used for push notifications.

[jshufro]

It's unlikely we'll branch out beyond the receivers supported by alertmanager, but you can run this project to relay them: https://github.com/pinpox/alertmanager-ntfy