support for custom animation

[iTonyYo]

If you can support custom animation, it will be more flexible.

Message.tsx#L93

[baleeds]

How would you feel about the library using react-transition-group, which would allow us to customize the animations using CSS. The react-spring seems like overkill for this use case, and it isn't very performant for me.

[baleeds]

Plus it's an extra dep, which just doesn't seem necessary.

[bmcmahen]

Happy to accept a PR for react-transition-group support, and agree that CSS animations would be preferable for an independent library. I built this for use within a project which included react-spring, so I wasn't worried about the extra dep.