add support for jack audio connection kit

[loblik]

Hi, I've created simple JACK backend for librespot. It's just a simple initial implementation. But it works quite good. It seems to be better than using portaudio (to access JACK) or pipe backend with jack-stdin.

Commit message follows.

This is initial support for JACK. It creates ports at startup and keeps it connected while librespot is running. So when librespot playback is stoped it writes silence (zeroes).

It uses jack crate (rust-jack) which needs libjack. To compile in jack support use --features jackaudio-backend. And run librespot with --backend jackaudio.