Replace handle_info_or_assign/2 with macro

[schrockwell]

My library live_event works similarly to this one. I used this defoverridable trick, which I learned from the Surface, to prevent the need for the user to explicitly add a hook in update/2

https://github.com/schrockwell/live_event/blob/1cdf78ca8e215c196c30e98f17d849c6e2c5b8bb/lib/live_event/live_component.ex#L29

Thought you might find it handy for a cleaner API in components. Would be happy to put together a PR if this change is desired.