add new sfvxxhash format

[borco]

Please add a new file format sfvxxhash that uses xxhash instead of crc32.

Links:

• homepage: https://xxhash.com/
• python package: https://pypi.org/project/xxhash/

[lxp]

Thanks for your interest in cfv. I will not personally have time to implement various new formats like also with #9 and #41. However, I gladly accept pull requests implementing new useful formats.