New compression formats

[diesl]

Hi @deitch

similar to #220 and #197, I want to propse a new compression format xz. Maybe I will open a PR, but I have a few questions before:

• Adding COMPRESS and UNCOMPRESS commands is quite easy, but how about more custom options? In this case, compression settings (-0 .. -9) or number of threads to use (-T) to name a few where a good default is hard to guess. Would you accept some COMPRESS_OPTIONS and DECOMPRESS_OPTIONS env variables?
• Thinking it further, would you be open to add a custom compression format, where you could define a custom COMPRESS_COMMAND and DECOMPRESS_COMMAND? It could of course be combined with previously mentioned COMPRESS_OPTIONS and DECOMPRESS_OPTIONS

Example:

COMPRESSION=xz
COMPRESS_OPTIONS=-z3 -T4
DECOMPRESS_OPTIONS=-d

COMPRESSION=custom
COMPRESS_COMMAND=xz -z3 -T4
DECOMPRESS_COMMAND=xz -d

[deitch]

I have no issue with new compression variants and options.

The issue with commands is that I am working to move this out of bash scripting into compiled (go). There have been too many issues with the logic in script, there are capabilities I want to add that are very difficult to do in shell, and I want to start offering this is a standalone binary on all OS/arch, not just a container.