Add an example of GIF animations

[yihui]

Per request of @thomasp85 https://twitter.com/thomasp85/status/923263990121353218

Something like this should work (requires FFmpeg):

```{r random, fig.show='animate', ffmpeg.format='gif', dev='jpeg'}
for (i in 1:10) plot(runif(100), ylim = c(0, 1))
```

[thomasp85]

Thanks - didn’t know ffmpeg made gifs - has been hacking it with magick