Implement transformItems API on widgets

[iam4x]

On every other InstantSearch flavours widgets have a simple API to perform any operation on the items before they are renderer. (ex: sort, map, ...)

From React-InstantSearch documentation:

Function to modify the items being displayed, e.g. for filtering or sorting them.
Takes an items as parameter and expects it back in return.

(items) => items.map(...)