Formalize Support for ParallelGZipOutputStream

[sstadick]

Hello!

Is there any particular reason that ParallelGZipOutputStream has not been formalized over the years? I find it incredibly useful, and wouldn't mind trying to put together a PR for supporting it, but I wanted to check if there was a good reason it hasn't been rolled in before?

https://github.com/haf/DotNetZip.Semverd/blob/master/src/Examples/C%23/ZLIB/ParallelGZipOutputStream.cs

[haf]

@sstadick Feel free to PR if you're using it and you think it's needed.