Add index as array support to fetchAll()

Using `[]` at the end of your index column declaration in `$query->fetchAll()` will return a multi-dimensional array within the index group.

Example `$query->fetchAll('sport[]')`
```
Array (
    [basketball] => Array(
        [0] => Array(
            [name] => John
            [age] => 18
            [sport] => basketball
        )
        ...
    )
    [football] => Array(
        [0] => Array(
            [name] => Jordan
            [age] => 24
            [sport] => football
        )
        [1] => Array(
            [name] => Emily
            [age] => 22
            [sport] => football
        )
        ...
    )
)
```