Otherwise it prints "Interrupt" and a traceback.

* Use a new, currently empty `Board#winner?` method as our loop
  condition
* Add `Player#insignia` so we can print the correct player insignia
  depending on who the current player is
* Print the board and a prompt for coordinates
* Switch the current player after a player finishes moving

Return the actual board in the right format instead of always returning
3 rows of 3 dots.

Also add `Board#update_cell` to facilitate player movement.

Add an `initialize` method to get rid of our test errors and use the
constants we created in 231060de4c493a9c6ba3510a9be4d405517af5c6 when
creating new `Player`s.