[feat] if quit stop vpn

As a side effect if the user shuts down the systray it will not
autostart next time.

- Related: #19

systray.go

@@ -48,6 +48,12 @@ func run(bm *bitmask.Bitmask, conf *systrayConfig) {
 }
 
 func (bt bmTray) onExit() {
+	status, _ := bt.bm.GetStatus()
+	if status != "off" {
+		ch := bt.bm.GetStatusCh()
+		bt.bm.StopVPN()
+		status = <-ch
+	}
 	log.Println("Closing systray")
 }