Support index reference in reduce function

[claytonflesher]

It would be nice, and a lot less verbose in some use cases, if we could support the index of a collection as an optional argument in the reduce function.

The implicit variable would be $$$ as it is in pluck, and if you named the variable it would be the third argument, making the new interface for reduce be:

(item, acc = [], index)

while still supporting the existing:
(item, acc = [])

By making the index an optional third argument that isn't defined unless referenced in the function call, it should make existing implementations backwards compatible while providing a useful variable for complex reduce use cases.

[machaval]

Agree this is very useful addition though it can not be added to the current reduce function without breaking compatibility, so this will require the introduction of a new reduce funtion like reduceWithIndex or something like this.