outline classes
add #initialize to Board
add #[] to Board
add #initialize to Piece
add #initialize to Piece
add Board::valid_pos?, Piece#slide_moves; rework Piece#initialize
add private Board#populate board, rework Board#initialize to call it
add require 'colorize', reorder Board methods
rework/fix Board#populate_board, add Board#render (needs work)
swap colors in Board#populate_board to match logic in Piece#slide_moves; rework slide_moves to include kings
rework Board#render, add @display, #promote to piece
rework Piece#slide_moves, add Piece#valid_moves, #jump_moves (in progress)
add grid print to Board#render
rename Piece#valid_moves to #shift_moves and rework logic. Validation belongs elsewhere.
add Board#perform_slide, InvalidMoveError
fix Board#perform_slide by removing Piece#shift_moves
refactor Board#perform_slide; make Board#[] private
refactor Piece#jump_moves and #slide_moves by adding #get_shift
add working Board#perform_jump
refactor Board#perform jump, add Board::coord_min_max