Extend _.sortBy to be able to sort by multiple fields

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 }]

// While it's backward compatible
_.sortBy(people, 'name');
// => [{ name : 'curly', age : 50 }, { name : 'curly', age : 30 }, { name : 'moe', age : 20 }]
```