Add an Explorer AI mode that favors nearby exploration

[QuintillusCFC]

The first iteration of the Explorer AI uses a greedy algorithm that seeks to reveal as much land as possible as quickly as possible. That is handy, but often results in the explorers going far away from home, because that's where the most unknown land is.

There should also be a mode where it favors uncovering tiles close to known tiles. This is more practical for early settling, and the first explorer (and perhaps second/third) should use this mode.

Adding to Carthage for now, but this falls into the "optional" category and could be postponed to Dutch.