Add Signal backend

The [Signal](https://whispersystems.org/) mobile messaging client would be a great fit for ntfy. Notably most of the current notification providers do not provide end-to-end encryption. There's already a convenient CLI client for Signal, [signal-cli](https://github.com/AsamK/signal-cli), that is simple to bootstrap and could handle the bulk of the logic for sending messages.

Assuming @dschep has no objections to calling the `signal-cli` directly, implementing a Signal provider in ntfy should be rather simple, wrapping those calls in a clean API.

Happy to work on this.