Added history — though still need to be checked thoroughly

Sources/Engine/Square.swift

@@ -75,6 +75,7 @@ public struct Square: Equatable {
 	public let position: Position
 	public internal(set) var piece: Piece? = nil
 	public let color: Color
+	public internal(set) var targetted: Set<Piece> = []
 
 	public static func == (lhs: Square, rhs: Square) -> Bool {
 		return lhs.position == rhs.position