Create a decorator to handle modifiable values

[mrroach]

program strength, cost, and other stuff are all currently repeating a pretty simple pattern that should be wrappable in a decorator. Something like:

class Program
  @property
  @Modifiable(modifiers.ProgramStrengthModifier)
  def strength(self):
    return self.STRENGTH

[mrroach]

decorator created but not yet universally applied (@modifiers.modifiable)