Handling pawn promotion

[captainsano]

The library at present does not handle pawn promotions. I'm planning to put up a PR for this. What would be an ideal way to provide APIs for this? My suggestion would be,

1 - Update onMovePiece to also pass the promotedToPiece param in case of promotion.
2 - Add a promotion prompt as soon as we recognize white/black pawn on 8/1 rank. (not API but displayed on the board)
3 - Cancel the promotion prompt if new props come in or if the user clicks outside.

A good start?