Add axes' labels for a dungeon map

[Cordarion the Grey]

Here are few thoughts about appearance:

1. Numeric offsets relative to the entry. Advantages: stable (do not change on map updates); can aid in treasure locating.
2. Chess-like. Advantages: the style used by LEM's visualizer; more concise notation (M2 vs 3E7N).
3. None, just display the map size (e.g. 9x12). The least intrusive way.

I'd personally prefer the first option. Any more ideas?