Adds a includeArrayIndex option to the $unwind stage. If this option is
specified, the $unwind stage will output values with the structure {index:
<array index>, value: <array value>} instead of just the array value.
Note this does not affect the behavior non-arrays or empty arrays. If
includeArrayIndex and preserveNullAndEmptyArrays are both specified, values
from a non-empty array will have an index attached, and all other values will
pass through unchanged.
