Promise support

Add promise support to allow mix different tasks:

async.waterfall([
    function (done) {
        // Do something and call done()
        fs.readFile(filepath, done);
    },
    function(content) {
        // Do something and return Promise
        return mongoose.models.file.create({
            name: filepath,
            content: content
        });
    }
], (err) => {
    if (err) {
        console.error(err);
    }
});

[nojvek]

I think just like there is async.asyncify there could be a async.promisify function.

Then I can just await async.promisify(async.mapLimit(x,10, mapper))

[eladnava]

Here’s another package that promisifies all the available async methods: https://github.com/eladnava/koa-async