Make `render` generate animations

[apiad]

When using render, allow for a possibly optional flag --animations that makes auditorium inject into the generated HTML the content for all frames of animations, and corresponding JavaScript to change them.

Beware that in general is it not possible to cover all possible cases, e.g., animations that have some randomness, animations that interact with input values, nested animations whose state depends on different combinations of frames.

Since this can potentially create very large HTML files, perhaps it should be opt-in and log a warning when animations are found and --animations is not passed.