.addAll() should return a promise

[mawrkus]

Hi,
First of all, thx for the library! I'm using it for a web scraper pet project...
A small detail, for the sake of consistency/convenience, I think that .addAll() should return a promise:

const oneTwoThree = promiseThrottle.addAll([
  myFunction.bind(this, 1),
  myFunction.bind(this, 2),
  myFunction.bind(this, 3)
]);

oneTwoThree.then(r => console.log("Promises " + r.join(", ") + " done"));

This should do the trick:

PromiseThrottle.prototype.addAll = function (promises) {
  var addedPromises = promises.map(function(promise) {
    return this.add(promise);
  }.bind(this));

  return Promise.all(addedPromises);
};

I can submit a PR if you agree on the idea.

[JMPerez]

@mawrkus Thanks for the feedback! I think it makes completely sense. Feel free to send a PR or I can make the change myself in a few days.

[JMPerez]

Fixed through #7