`concurrency` should detect available concurrency levels by default

[miketheman]

Having a default set at 10 is great, but getting the available cores is even better:

const os = require('os');
const cores = os.availableParallelism(); 
console.log(`Available parallelism: ${cores}`);

Ref: https://nodejs.org/api/os.html#osavailableparallelism