Add support for custom keyframe animations.

Right now, the animation on flash is a simple CSS transition (defined here: https://github.com/lab49/react-value-flash/blob/main/src/Flash.tsx#L86).

We could add the ability for users to provide their own custom keyframe animations. You could likely do this already by writing your own CSS and using the selectors in the DOM, but perhaps there's an easier API that we could provide.