Calgary Challenge update

[Matt Mahoney]

Effective June 1, 2017, I am adding 7zip and zpaq to the list of allowed archive formats.
http://mattmahoney.net/dc/calgary.html

I haven't done any development work on the challenge (or on zpaq since the last release in Aug. 2016), although I might in the future.

I made the decompression program optional. I think that a custom compression program could still beat the best zpaq model, but maybe not enough to overcome the size of the decompression program. It remains to be seen.

The older formats are still allowed (unzip, unrar, unbzip2 and ppmd var. I). I am only keeping ppmd-I because previous submissions used it.