Extend _.sortBy to accept iterators that return arrays

I think the `_.sortBy` can become more flexible if it can accept an iterator that returns an array.
This can be useful is some cases such as sorting tables of data by more than one column.

```javascript
var people = [
    { name : 'curly', age : 50 },
    { name : 'moe', age : 20 },
    { name : 'curly', age : 30 }
];

_.sortBy(people, ['name', 'age']);
// => [{ name : 'curly', age : 30 }, { name : 'curly', age : 50 }, { name : 'moe', age : 20 }]

_.sortBy(people, function (person) { return [person.name.length, person.age] });
// => [{ name : 'curly', age : 30 }, { name : 'curly', age : 50 }, { name : 'moe', age : 20 }]
```