Feature request: merge multiple async iterators

[pietercolpaert]

A function merge on an AsyncIterator would be a great tool to e.g., merge sort different iterators:

require('async-iterator').merge([iterators], function (a, b) { return a.departureTime < b.departureTime})

Which element is emitted next is based on a boolean condition.

[RubenVerborgh]

I think @rubensworks made a library that does this. Might be able to steal that code.

[rubensworks]

Yes, I made asynciterator-union, which takes the union of two streams, but it doesn't do sorting.
It should not be too hard to extend it with a comparator-based iterator though.

(Note to self: we may need a new abstraction, where both RoundRobinUnionIterator and MergeSortUnionIterator extend from it)

[RubenVerborgh]

Now also available in this library itself: f405a7d