Unflatten into objects with integer keys

Added `object` option to unflatten. When set to true, numbered keys are unflatten into objects with integer keys (instead of always creating arrays).

```javascript
unflatten({
    'hello.you.0': 'ipsum',
    'hello.you.1': 'lorem',
    'hello.other.world': 'foo'
}, { object: true })

// =>

hello: {
    you: {
        0: 'ipsum',
        1: 'lorem',
    },
    other: { world: 'foo' }
}
```