Support callback in setState

[jossmac]

No description provided.

[banderson]

@jossmac since setState is synchronous, you can just call your function right after your setState call instead of passing a callback.

[sindresorhus]

@banderson setState can be async: https://reactjs.org/docs/state-and-lifecycle.html#state-updates-may-be-asynchronous

[jamiebuilds]

MINES BETTERRRRR

Closing in favor of #28