add --format=pigz & --format=pxz multi-threaded support

[centminmod]

I just finished some zstd 1.4.0 benchmarks including testing --format=gzip and --format=xz options and they seem to be single threaded. Would love to see zstd support multithreaded gzip via pigz and multithreaded xz via pxz as well for --format=pigz & --format=pxz :)

[terrelln]

I didn't look that closely, but it looks like those tools don't provide libraries, only command line tools.

Zstd uses the zlib and lzma libraries to provide support for --format=gzip and --format=xz. We'd have to shell out to the the respective CLIs to support pigz and pxz. I'd rather not do that.

If there widely used library for parallel xz or parallel zlib, I'd be happy to accept a PR that adds support.

[centminmod]

cheers @terrelln wouldn't I don't know inner workings of pigz or pxz or if there are such libraries or not.

I believe pigz issue was posted at madler/pigz#50

[Cyan4973]

Closing. Such a feature would require advanced dependencies, that we prefer not to depend upon.