Introduce List Options with `recursive` flag

[abronan]

There was no real alignment on the List call. The list of children would be different from a store to the next, with one returning the recursive version and the other not, or one returning the directory key and the other not for example.

#32 attempts at aligning the List call for every backend with the recursive use case. Although this is not viable for very large sets of keys and subfolders.

We should introduce a recursive option to Listto be able to select whether we want the result to be recursive or not. This could be a simple boolean flag for example.

[nmengin]

@abronan LGTM :wink: :loop: :+1: