Limit number of concurrent tasks in async.auto

[vergenzt]

I'm using async.auto to spawn some subprocesses for tasks in a dependency tree. Right now it just runs as many tasks as it can run at a time, but I'd like to add an option to limit the total amount of tasks running at a given time, in the same way that async.queue currently works.

[vergenzt]

Working on a PR for this.

[cryptoquick]

I needed this functionality for this utility I'm making:
https://github.com/cryptoquick/qonsumer
It works very well so far, so I give it my thumbs up! 👍

[vergenzt]

Thanks for the support! 😃

[cryptoquick]

👍