Add refreshInterval option to identity requests

Hi.

As I understand, there is no way to do this right now. Is it possible to add this feature?
I think it would give more flexibility, especially paired with https://github.com/heroku/react-refetch/issues/177

```js
const fetchUser = id => fetch(`/users/${id}`);

connect(props => ({
user: {
value: () => fetchUser(props.match.params.id),
refreshInterval: 60000
}
}))(Profile);
```

In this example, function provided to `value` should be called every minute.