Add byes and average byes per match to the Top Wicket Keeping report.

From @Scott Walker :

I used to have a lot of fun sharing byes statistics on the old program when players became a little critical of my keeping. I used to export other more specific WK reports, copy it to excel and work out byes per game for each player with the AVG formula. I just went searching on the web version and can't seem to find them anywhere.