More control over formation changes.

Use player names in formation to have more control over what player moves to a new position.
The old way a single change in formation would cause a lot of movement and often leave the team vulnerable.

    def players
      {
        average: [:pepe, :andres, :luis, :juan, :jorge, :german],
        fast:    [:gaston, :pedro, :marcos],
        captain: [:nelson]
      }
    end

    def formation(match)
      formation.lineup = [
        [:pepe,  :none,  :andres, :none,   :none],
        [:luis,  :none,  :none,   :none,   :gaston],
        [:none,  :pedro, :none,   :nelson, :none],
        [:juan,  :none,  :none,   :none,   :marcos],
        [:jorge, :none,  :german, :none,   :none],
      ]
    end
I understand that we are checking the players being defined in the #players method now. We're not longer checking the formation, hence the method name confuses me. What about using the names ```players_errors```  / ```players_valid?``` or similar?