Windows support

Thank you for this package, been following your module-request since the start. I was also trying to implement this but never got around to finish it, this seems like a solid solution though!

I took some time to look for a Windows (there are dozens of us) solution and found a sysinternals utility that tries to mimic the du command.

A small problem here is that it returns the actual size of the (child-)directories and not the disk-size of these directories, which leads to different results (see image below, should be ~130MB total (on disk)).

It can return the disk-size, but only for one (the top-level) directory, so I assume there should be an extra loop in the Windows .js util by getting the child-directories and sizes individually.

Any thoughts on this? A dependency too much (.exe)? A loop too many?