Add perform_jump and perform_slide
Move tile formatting to separate method
Fix bugs in Piece sliding and jumping
Add Board#on_board?
Refactor
Create InvalidMoveError
Add Piece#perform_moves!
Add Piece#perform_moves
Move sliding and jumping methods to private
Add coords to side of Board display
Create HumanPlayer
Fix bug in Piece#intermediate_position
Create Board#move that calls perform_moves on a piece