Feature request: Write NPZ to memory

[zbendefy]

Hi!

I'm looking for a feature to be able to write the NPZ content into a memory stream (as it is already possible using npy export).

I have a basic implementation, but its just a first attempt to see it working, without much consideration of the interface.
https://github.com/zbendefy/libnpy

If you have some comments or preferred direction of how to include this feature, I would apply them and would submit this as a PR.

Thanks