Add ability to 'flash' selections

I really liked the ability of the old editor to flash selections at you when you cycled through the find results. This is an attempt at adding the same functionality to the react editor.

I want the flash to be temporary, so I'm using the same mechanism as the autoscroll:

```coffee
editor.setSelectedBufferRange(marker.getBufferRange(), autoscroll: true, highlighted: true)
```

It's a bit awkward, though. @nathansobo take a look and let me know if you think this is on the right track.

Any way to do this without these spies? I'd rather we used the animation frame mocking and just rely on inspecting the state of the class list.