Add option to toggle extra level logging.

Previously, we would record the current level every time the player
gained XP. This is a lot of unnecessary data, and a few levels' worth of
data indicates that it is not necessary, as our mechanism of storing and
backtracking to the last level-up event works fine.

Rather than remove the code altogether, I've added a new configuration
option to control it. (This is an excuse to try out the config registry
as much as it is useful functionality.)