I'm a heavy user of async.queue, but if I am using it to queue calls to an API service which implements rate limiting (say 5 API calls per second), it becomes very tricky to get the queue to consume the queue within the rate limits.

To fix this, I branched the code and added a new parameter to the queue object:

var worker = function(data, done) {
  setTimeout(done, 100);
};
var concurrency = 1;
var rateLimit = 5;
var q = async.queue(worker, concurrency, rateLimit);

The queue works as normal but only proceeds at a maximum rate of 5 items per second.

The existing tests all pass and supplying a rateLimit of null or leaving it as undefined keeps the original queue unchanged.

I've added a test for the rate-limited version.

(as this is my first commit to this project, I'm not sure if I should be including the "dist" files in the PR - please let me know if you would like these included too).