Add custom factory support for nest

[nyurik]

A more generic approach to customize new node creation using a factory method.

This allows (for example) to create sub-aggregations for each node of all sub-nodes without doing a complex postprocessing (no need to implement tree iteration)

d3.nest().factory((k, v) => (
  {
    key: k,
    values: v,
    sum: v.reduce((acc, vv) => acc + vv.sum, 0)
  }
).key( v => v.foo ).entries([
  {foo: 'a', sum: 10},
  {foo: 'b', sum: 15},
  ...
]).