Added threading support

[mlimber]

Fulfilling issue described in issue 203.

[megastep]

This is interesting, but my main concern is specifying these threads arguments to commands that will then be invoked by the archive. It seems to me that it would make such archives more prone to fail to extract on systems where the xz command for instance isn't compiled with threading support.

[megastep]

Also the benefits of these I would think are fairly minimal for most systems, and would only have a small performance improvement for massive archives (where I/O tends to be a bigger bottleneck anyway). So I'm not convinced potentially breaking the universality of archives is a good trade-off.

[megastep]

Nevermind, looks like I misread the code and was thinking of decompression more than compression 😅

[mlimber]

Thanks. I upped the version number to 2.4.2. Do you want to create a release tag for it? (Note: the version log at the top of the main .sh file didn't have an entry for 2.4.1.)

[megastep]

Probably soon. There's a few other issues that might warrant another look first.