Turn TicTacToe::Game into a class

I am going to want to refactor this code and add extra functions, but
currently it looks like most of that would require passing around board
and player objects everywhere.  Pulling these attributes up into a constructor
method and defining a reader gives us the flexibility to access them across the
whole object without having to pass them to the helper functions we're about to
create.

app.rb

@@ -1,3 +1,4 @@
 require_relative "lib/tictactoe"
 
-TicTacToe::Game.play
+game = TicTacToe::Game.new
+game.play

lib/tictactoe/game.rb

@@ -1,17 +1,19 @@
 module TicTacToe
-  module Game
-    extend self
+  class Game
 
-    def play
-      board   = [[nil,nil,nil],
-                 [nil,nil,nil],
-                 [nil,nil,nil]]
+    def initialize
+      @board   = [[nil,nil,nil],
+                  [nil,nil,nil],
+                  [nil,nil,nil]]
 
-      left_diagonal = [[0,0],[1,1],[2,2]]
-      right_diagonal = [[2,0],[1,1],[0,2]]
+      @players = [:X, :O].cycle
+    end
 
+    attr_reader :board, :players
 
-      players = [:X, :O].cycle
+    def play
+      left_diagonal = [[0,0],[1,1],[2,2]]
+      right_diagonal = [[2,0],[1,1],[0,2]]
 
       current_player = players.next