feat: zstd compression and compressor levels

[erikgeiser]

This PR closes #66 by adding support for zstd compression. It also allows users to optionally specify the compression level for gzip (gzip:-2 up to gzip:9) and zstd (zstd:default, zstd:fastest, zstd:better, zstd:best, zstd:17). It also contains a test for the compression configuration.

I can also add bzip2 such that this PR can supersede #67 if you want.

[jarondl]

Thank you very much for the contribution, along with good testing!

[erikgeiser]

Thanks for your review. I fixed the issues. Should I also add bzip2 support such that this PR can supersede #67?

[jarondl]

Yes, please add bzip2, seems simple enough to combine here.

[jarondl]

/gcbrun

[erikgeiser]

I took a look at bzip2 but I found that the library used in #67 seems like a stale repository where all commits are from 2015 and 2016 and no issue was ever closed. I also did not found a better library. I don't really want to take the responsibility of adding this stale dependency to this project.

Thank you for merging this PR, though!