New Functionality: Input Keys from a File to Download

HI,

Can we have this additional functionality where list of keys to be downloaded can be provided via a file through an additional argument and for simplicity this input file can be an other Blob as well.

Use case, we have millions of small files(images) which are required for training on a need basis, so it would be very handy for this use case.

Right-now am relying on Python multiprocessing for the same but I think Go would be much faster based on the experience using your module.

Thanks