Add --on{start,stop} command line option

The --onstart and --onstop command line options can be used to run a
program when the audio playback is about to begin or has ended.

Note, that librespot needs executions rights to run the program.
Furthermore, the full path needs to be specified, e.g.
`/usr/bin/logger`. Executable scripts must begin with a shebang, e.g.
`#!/bin/sh`.

src/main.rs

@@ -26,6 +26,7 @@ fn main() {
     main_helper::add_session_arguments(&mut opts);
     main_helper::add_authentication_arguments(&mut opts);
     main_helper::add_player_arguments(&mut opts);
+    main_helper::add_program_arguments(&mut opts);
 
     let args: Vec<String> = std::env::args().collect();