Add ability to turn codecs on and off

[nemequ]

From @nemequ on August 13, 2015 8:54

The copy codec can be a useful point of reference, but it can also mess up the scales because it can be a huge outlier. It would be nice to be able to turn the codec on and off at will.

This functionality could/should also be extended to all codecs, and perhaps even each level. That would allow people to, for example, only compare a subset of codecs (e.g., people probably aren't interested in ZPAQ and density).

Copied from original issue: quixdb/squash-benchmark#23