Use arbitrary tree child node key name

[jimjoes]

As far as I can tell, a tree data object must use the convention that child nodes are all in the nodes key. My API uses a different key. It would be great to be able to be able to a) specify a child/parent relationship for a flat list and have the library assemble the tree according to arbitrary id/parentId fields and/or b) specify an arbitrary key for node children in a nested tree.