Support Transition prop "disableIn" and "disableOut"; or some way to skip LeaveFrom

[mcgrealife]

Why: LeaveFrom always executes, even if transition-none – "flashing" an empty dialog frame some milliseconds.

My use:

• I have a generic modal component, which accepts a prop to control transitions conditionally.
• I use the prop in the Transition's enter and leave props, to set transition-none or transition-all.
• However, even with transition-none, the component still adds the leaveFrom class. So for a millisecond, a frame is shown with an empty dialog.

Current workaround:

• use conditional classes in the leaveFrom, so that leaveFrom uses the same class as leaveTo.

Preferred solution:

• A simple disableIn or disableOut prop that could skip the leaveFrom