Added setdisabledbydefault method

Needed this for a project I'm on where I need to enable the transitions on the init of a class, and disable them on the teardown. In this case, I don't necessarily know if 'disabledbydefault' is true or false, so simply toggling it wasn't enough — I needed an explicit setter.

My solution was just to write a setter, but it might be better to just expose this simple value on the $ object rather than manage it with two different methods.