feature request: support pick item props

[atian25]

const tree = arrayToTree(arr, {
  transform: ['id',  'key1'],
  // transform: item => { return { id: item.id, key1: item.key1 } },
});

[gburning]

I would appreciate something like this as well. I don't need parentId once my collection is already hierarchical tree for example. It would be nice to be able to strip it from the results.