Align Container#setState closer to React's implementation

[jamiebuilds]

This is an initial attempt at making Unstated's Container#setState work more like React's Component#setState.

setState is async

this.setState({ value: 'foo' });
this.state.value; // 'prevValue'

updater can be a function

this.setState(prevState => ({ counter: prevState.counter + 1 }));

An optional callback will run after the component's are updated

this.setState({}, () => {
  // components are updated
});

[xzilja]

This probably addresses #1 and #27 ?

[sindresorhus]

I've tried this PR in our app and everything seemed to work fine.