edited Piece move method
added ability to move pieces on the board
added ability for king to castle
made king move on castle
made the rook move on castling too
cleaned up move function
allowed players to move only their pieces
corrected Black King icon
added relative square helper function
added method to check if a piece is an opponent
made case irrelevant for move method
added color method to string
fixed opponent? method
added ability to capture en passant
cleared some empty lines
allowed player to retry move if invalid entry
added rank and file indicators on all sides of the board
added quit, exit, and q commands for player
Update README.md
wrote test for check?