advance_runners function is overly simple

The advance_runners function advances runners based on the base the hitter gets to (on a double, everyone moves forward two bases). But, for singles especially, decent runners can go from 1st to 3rd on a single. Perhaps there should be a "time to advance bases" concept where runners have a certain amount of time to advance based on the hit type? This is related to issue #11 as it lowers the overall score.