Worker ID

Would it be possible to add args here:

https://github.com/andrewjstone/forkqueue/blob/master/queue.js#L25

var worker = cp.fork(this.workerModule,[this.workers.length]);

so that the worker process can get his ID from progess.argv like this:

const ID= process.argv[2];

this would enable better logging:

console.log('builder['+ID+']: '+message);

Thanks, great job
