Add autoPlay events: play, stop, pause, unpause

I have a playpause button, which I'd like to update the icon of, when the player paused state has changed.

Currently I cannot find any events for the player, and I'd like to suggest adding `play`, `stop`, `pause`, and `unpause` events.

For example:

```
// jQuery
$carousel.on( 'play.flickity', function() {
console.log( 'Flickity player playing')
})
// vanilla JS
flkty.on( 'play', function() {...})
```

Start by tracing Flickity's autoPlay/player behavior and how existing events are exposed to jQuery and vanilla JavaScript consumers. Define the play, stop, pause, and unpause event points, then verify that each event fires when the corresponding player state changes.

Written by the indexing model from the issue text.