Add ability to use Skill Modes

[stevenvergenz]

In Fate Accelerated and Atomic Robo, the skill system is less detailed than in Fate Core. Skills are grouped into Modes, and you choose a mode to be ranked at 3, a mode at 2, and a mode at 1. A skill is the same level as the highest mode containing it, plus one level for each additional mode. You also have a small pool of points to spend to upgrade certain skills.