Challenge games

A note on difficulty: I actually think repair costs are too low. I almost never forgo repair to save money for weapons or ships. I think the decision to repair or not repair has minimal strategic consideration, because it's outweighed so easily by income. Most of that strategic value is also limited to the early-game, when scrap is harder to come by.

I think the repair costs should be more heavily impacted by difficulty levels, if they are currently impacted, but also raised somewhat at easy. (Haven't done a careful comparison.)