Exit gracefully on SIGTERM

* Exit gracefully on `SIGTERM`

Save current state and close ncspot on `SIGTERM`

Fixes #948

* Disable signal handling on non-UNIX platforms

Cargo.toml

@@ -46,9 +46,13 @@ toml = "0.5"
 unicode-width = "0.1.9"
 url = "2.2"
 cursive_buffered_backend = "0.6.1"
+
 [target.'cfg(target_os = "linux")'.dependencies]
 wl-clipboard-rs = {version = "0.7", optional = true}
 
+[target.'cfg(unix)'.dependencies]
+signal-hook = "0.3.0"
+
 [dependencies.rspotify]
 default-features = false
 features = ["client-ureq", "ureq-rustls-tls"]