Add Kino.Frame for animations

[jonatanklosko]

Closes #48. For the Livebook counterpart see livebook-dev/livebook#688.

This introduces a new dynamic output that wraps a single static output, which can be replaced at any time. Additionally, we now have Kino.animate wrapping the most common use case.

kino_frame.mp4

Sidenote: Kino.animate is how to flip book in Livebook

[josevalim]

I love the name and the abstraction!

Some concerns:

1. Is it too much to have both the public module and Kino.animate? Shall we @moduledoc false this one? Or do we need to keep it public because the result of Kino.animate is public?
2. Kino.render does not return an output. Kino.VegaLite.periodically only returns ok. The fact animate does is a bit inconsistent?
3. Should we rename Kino.VegaLite.periodically to Kino.VegaLite.animate for consistency?

[jonatanklosko]

@josevalim

1. We still want Kino.Frame to be public, because someone may use it explicitly, specifically the Kino.Frame.render function. This can be useful if the rendering is not periodical, but rather based on some event, just to give an example, someone may re-render output from an Axon callback.
2. Ah, initially I thought we want to return the widget to render it, but you're absolutely right, we should call Kino.render from Kino.animate, so it works anywhere in the cell.
3. For the widgets, periodically is actually more accurate. The callback we pass to periodically doesn't return any new data to animate, it needs to be done using Kino.VegaLite.{push,push_many} or Kino.Frame.render.