Add SliderPiece#diag_moves.
Prettify SliderPiece#rect_moves.
Unify #knight_moves and #king_moves.
Populate board squashed awful bug.
Fix board display and add to ChessGame.
Add HumanPlayer class in separate file.
Prettify, colorize pieces and board.
Move validations for user input.
Added axes, fixed bugs.
Fix position memory for moves.
Remove jumps for all but Knights.
Fix ... some bug.
Finished encapsulation and moved some things to new files.
Split classes into separate files.