Make decorators optional in config files

This is to avoid having every addition / deletion of a new decorator require edits to every configuration file. With this change every decorator is off by default.

We'll also check to make sure the decorators that are enabled (or disabled) are valid. If they are not, we raise an exception.

This is related to #37 (closed), but still leaves it open. I should ping @pgadow since I think he was about to add yet another decoration alg that we need in a very specific case.