Add thunk support - #109

@developit I saw you thumbs upped this, would it be able to get merged in?

Can't this already be done with Unistore's existing async action functions?
Here's a rewrite of your example above (I've kept the function name thunkAction):

let helpers = {
 setUser: (val) => ({ user: val }),
 setMessage: (val) => ({ message: val }),
};
let actions = store => ({
 simpleAction: (state, value) => ({ count: state.count + 1 }),
 async thunkAction(state, value) {
 let response = await fetch('/foo.json').then(res => res.json());
 const updatedState = {
 ...helpers.setUser(response),
 ...helpers.setMessage('Thanks!'),
 };
 return updatedState; // or with store.setState(updatedState)
 },
});

Isn't the async flow equivalent to this PR's "thunk'ed" approach?

Also, in your example action is invoked twice (for setUser and setMessage), so I am wondering if this is to make a particular point, or just a random example? (as you can see in my modified code snippet above, I combined the state update into a single object)