add support for jack audio connection kit

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.

src/lib.rs

@@ -34,6 +34,9 @@ extern crate portaudio_rs;
 #[cfg(feature = "libpulse-sys")]
 extern crate libpulse_sys;
 
+#[cfg(feature = "jackaudio-backend")]
+extern crate jack;
+
 pub mod audio_backend;
 pub mod discovery;
 pub mod keymaster;