Add promise middleware

[yamadapc]

Promise middleware is a redux middleware that allows:

function myAction() {
  return {
    payload: {
      promise: fetch('/api/something').then(res => res.json()),
    },
    type: 'SOMETHING',
  };
}

Instead of:

function myAction() {
  return ({ dispatch }) => {
    dispatch({ type: 'SOMETHING_START' });
    fetch('/api/something').then(res => res.json()).then(
      (json) => dispatch({ type: 'SOMETHING_SUCCESS', payload: json }),
      (err) => dispatch({ type: 'SOMETHING_ERROR', payload: err })
    );
  };
}

[yamadapc]

It allows DRYer promise based code.