Pass this.state.data to then(), catch(), andThen() and andCatch() functions for more powerful side effects

Does it make any sense to have `this.state.data` available in those handlers? In my case, it's relevant for implementing an infinite scroller, where each fetch **adds** to the previous data and I need to somehow merge it.

Here's how that would look like:

```jsx
const connections = {
widgets: '/api/widget/?limit=20',
loadMore: offset => ({
loadingMore: {
url: `/api/widget/?limit=20&offset=${offset}`,
andThen: (res, meta, data) => ({
widgets: { value: data.widgets.value.concat(res) },
}),
},
}),
};
```

My current workaround is to pass a `getData()` function to `loadMore()` from the component, which works, but isn't nice.

On another note, I would like to have that `data` even in direct mapping functions (`loadMore()`above), so I can do things like optimistic updates.

And, 👍 for this very nice library, thank you for writing it!