Make equal sign (=) optional when loading pgn file
Make equal sign (=) optional when loading pgn file

Found some pgn files exported for xboard that do not use the equal sign
when promoting a pawn. Allow for parsing promotion without the sign

akshayverma2804/chess.js,ornicar/chess.js,r9k/chess.js,jeffthemaximum/chess.js,mattyhead/chess.js,linrock/chess.js,Assios/chess.js,jhlywa/chess.js,ornicar/chess.js