Add Animated PNG (.apng) as output format

This can currently be done by doing it in two steps:

```sh
puppeteer-lottie -i input.json -o 'frame-%d.png'
ffmpeg -r 30 -i frame-%d.png output.apng
```

But it would be nice if the CLI could do this automatically ☺️ 

Also, I'm not exactly sure about the framerate. It would be nice if it would be customisable so that we can get a 60-fps animated png file (maybe related to #2)