[Feature request] `duration.asObject()`

Hi,

It would be nice to have an `asObject` method on durations, e.g. to convert a number of milliseconds into `{ years, months, weeks, days, hours, minutes, seconds }`.

That would be a shorthand for the following :

```js
const durationAsObject = duration => ({
years: duration.years(),
months: duration.months(),
days: duration.days(),
hours: duration.hours(),
minutes: duration.minutes(),
seconds: duration.seconds()
});
```

Thanks