Mark (E)xplored on Overmap

This adds a new option to the overworld map that allows players to mark locations as 'explored'. This turns the symbol color for a marked tile to dark gray while overlays or blinking is enabled. This provides a more at-a-glance representation of places you have and haven't cleared out yet.

![Before & After](https://cloud.githubusercontent.com/assets/3086301/4435479/92225ece-4746-11e4-9b35-d4ce34543d6d.png)

This is the result of a morning of looking through this project's code and it's also the first time I've really done anything in C++ (I normally program in Java/C#/.NET), so if anything needs changing, please let me know and I'll be happy to do so.