Composing requests within the connect function (enhancement)

I can see in the docs the ability to use `PromiseState.all` in the component to put together a number of existing Promise states on the props. This is cool, however I was thinking it would be particularly neat if it could be abstracted out to the `connect` function such that a composed `PromiseState` could be directly passed down into the props from there.

For example:
```js
connect(props => ({
usersFetch: props.users.map(user => `/users/${user.id}`)
}))(App)
```
Which is then interacted with in the component `App` like so:
```js
render() {
return this.props.usersFetch.fulfilled &&
this.props.usersFetch.value.map(user => ())
}
```

Here it defaults the composed `PromiseState` to the completion of `all` of the contained PromiseState as opposed to the `race` option (can't think off the top of my head of a way to allow this to be nicely configurable though, since passing the array is the neatest and most simplistic means at the moment)

Would love to contribute to this project, so if this is a good idea and we can agree on implementation I'd happily code up a PR 👍