enhancement: add signal for messages

[peedy2495]

I'm a great fan of signal and never used telegram.

For this type of messaging there exists already a awesome project: AsamK/signal-cli
Y're able, to add new devices via qr-code, too. In bash: sudo signal-cli link -n "optional device name" | tee >(xargs -L 1 qrencode -t utf8)

Adding this enhancement would be gorgeous.

[tboerger]

IMHO out should only support alert methods that can get implemented within the go service without the need to shell out some command, but these are just my 2 cents.