It would be nice to be able to set 'concurrency' when creating `batch` instance

[MeoMix]

Currently I do:

      const batch = new Batch();
      batch.concurrency(20);

I would like to have the ability to do:

      const batch = new Batch({ concurrency: 20 });

[dougwilson]

This is a good idea :+1: Would you be interested in making a pull request to.implement this?

[MeoMix]

Yeah, sure. I can try to whip something together ASAP.

[MeoMix]

wow, past me was an overly optimistic jerk for saying he'd put up a PR and not doing it.

thanks for finally getting around to this feature! great job :)