watch should work only in when the runner is in idle mode

[ranyitz]

Users use the watch function when writing presets.

watch(`./**/*.scss`, changed => run(
    read({pattern: changed}),
    sass(),
    write({target: 'dist'}),
  ));

This watch is only important when the initial run has finished, otherwise, it could result in an unexpected behavior. (another sass compilation as a result of a copy tast on the first run)

If we'll start the watch listener only after the runner reached to idle mode, then the user would be able to configure the watch wherever he wants.