Add ability to listen (tap) the input

By doing something like
```
fn input_output_loop(input_output: Subject(List(String))) -> Nil {
  let output = process.receive_forever(input_output)

  echo output

  input_output_loop(input_output)
}
```

src/mpv/mpv.gleam

@@ -102,7 +102,7 @@ fn handle_message(
 /// This is useful to ultimately create a `Control` without the user having to
 /// input all of the character(s) needed.
 fn read_input(
-  subject: Subject(Control),
+  agent: Subject(Control),
   inject_input: Subject(Key),
   tap_input: Subject(List(String)),
 ) -> Nil {
@@ -111,10 +111,13 @@ fn read_input(
     Ok(_) | Error(_) -> []
   }
 
-  let _ =
+  case
     key.read_input_until_key(buffer, tap_input)
     |> control.from_key
-    |> result.map(process.send(subject, _))
+  {
+    Error(_) -> Nil
+    Ok(control) -> process.send(agent, control)
+  }
 
-  read_input(subject, inject_input, tap_input)
+  read_input(agent, inject_input, tap_input)
 }