Add zpaq support

[ci-macpaw]

Originally reported on Google Code with ID 823

josh.jpenguin:

zpaq is a Public Domain format

http://mattmahoney.net/dc/zpaq.html

[ci-macpaw]

paracelsus:

Oh, it changed license? I looked at it before and it was GPL licensed, making it incompatible
with The Unarchiver and fairly useless, but now it might actually be of interest.

I hope I can find some time to look into adding it.