Support for normalized data

[cdtinney]

I have a use case where there can be very deeply nested data that is loaded asynchronously into a Redux store, and is normalized.

e.g.

entities = {
  byId: {
    key1: {...},
    key2: {...},
  },
  allIds: [key1, key2]
}

It would be great if this had first-class support for normalized data.

Perhaps a childAccessor function could be passed as a prop that is called to access the child object when constructing the TreeNode.