By Frankie Taddeo Josh Allen is the SI Sportsbook favorite to be the postseason passing yards leader, ahead of Patrick Mahomes. With the kickoff to the NFL playoffs only days away, SI Sportsbook is adding intriguing futures markets for sports bettors. Outside of Super Bowl futures, one of the most popular offerings involves predicting which quarterbacks will lead the league in postseason passing yards. From a sportsbook perspective, the handle on these types of offering continues to soar as bettors look to invest on either quarterbacks from their favorite team or backing those whom they believ…