Add ability to listen to input

src/musicplayer.gleam

@@ -1,5 +1,8 @@
-import gleam/io
+import gleam/erlang/process
+import mpv/mpv
 
 pub fn main() -> Nil {
-  io.println("musicplayer")
+  let exit = process.new_subject()
+  let assert Ok(_) = mpv.new(exit)
+  process.receive_forever(exit)
 }