Support array of promises

If the lambda function returns an array, this module doesn't treat it as a promise and will just do `cb` on the pending promises. I suggest:

```
if (Array.isArray(v)) {
Promise.all(v).then(function (val) /* etc. */
}
```

We can also just wrap `v` in `Promise.all` regardless since it will work even for non-arrays. It would be slightly less efficient to do this. There is also no guarantee that there is a promise in the array.