Enhancement: Softcode strumline fade in to be disabled by modules

[manyshrimp]

As it currently stands, the code that runs fadeInArrows() on both strumlines is only dictated by PlayStatePlaylist.isStoryMode, as seen in this snippet from PlayState in initStrumlines():

if (!PlayStatePlaylist.isStoryMode)
{
  playerStrumline.fadeInArrows();
  opponentStrumline.fadeInArrows();
}

Due to the nature of tweens and the fact that this runs immediately on creation of the state, changing the individual position of notes within the strumline is difficult or, at the very least, inconvenient. I feel as though it would make more sense for this animation to occur when the countdown starts, with a public boolean to disable/skip it, to allow modders to more easily reposition arrows or implement their own animation that doesn't conflict with the original one. I would make this a pull request, but I'm not confident enough in my programming abilities for that. Someone else very easily could, however.