[feature request] bzip3 support

I don't see many people using it (yet), but bzip3 looks pretty impressive from the claims in the README:

https://github.com/kspalaiologos/bzip3

If the claims are accurate, then I expect that we'll see people switching away from bz2 and xz to bz3. It would be good to support bz3 in that case. So, opening a place holder bug here. :)