Allow end_pause when looping

[friendly]

This is related to #101, but my request is simpler.

In a looping animation, it is hard to see where it starts and ends. Absent any controls for the GIF, what is most needed is to allow an option end_pause = sec that would insert a pause of sec seconds before the gif repeats.

[kornelski]

For now you can duplicate the last frame lots of times.

[friendly]

OK, but if the software I use to generate GIFs doesn't provide for this, somehow I have to edit the GIF manually to duplicate that frame.

I think gganimate may allow this, but I'm using the tourr package, render_gif)