Add stream-size option to pzstd

[rossburton]

The standard zstd tool has a --stream-size option so the file size can be set for streamed compression. However, pzstd doesn't support this option, so any file compressed with pzstd has to be decompressed with a stream (so, eg, the PyPi zstd module can't be used).

[terrelln]

I'd accept a PR, with tests, that adds this. However, pzstd is not under active development, and it is generally suggested to move to native zstd multithreading through zstd -T#. We are still committed to fixing bugs, but aren't planning future features.

[andyluo03]

Could I still tackle this as a first issue? (is it still open?)