WIP add ability to 'flash' selections (find and replace used to do this)

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:

```
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.

Rather than the new `updateDecorationForMarker()` I _could_ remove the old and add the new. What do you think @nathansobo?