A copy of the vanilla GenLayerAddMushroomIsland with an optional disable. I can't 
think of a single reason to use this rather than just not adding the layer.



Constructor:
    Arguments:
        long, GenLayer
    Argument Usage:
        base layer seed, parent layer
    Notes:
        Constructs the layer with the seed and parent.

shouldDisableMushroomIslandLayer:
    Arguments:
        boolean
    Default Values:
        false
    Notes:
        Sets whether the layer should be disabled