Wrapper component

[robinweser]

This PR adds a Wrapper component that is a convenient shorthand for the Provider, Consumer pair when working with local state.

const Counter = createStore({
  model: 0,
  actions: {
    increment: state => state + 1,
    decrement: state => state - 1,
  }
})

const Tree = () => (
  < Counter.Wrapper initialState={5}>
    {({ state, actions }) => (
      <div>
        <div>{state}</div>
        <button onClick={actions.increment}>+</button>
        <button onClick={actions.decrement}>-</button>
      </div>
    )}
  </Counter.Wrapper>
)