Return the last activation time, earlierthan the given time.

[wenxinnnnn]

What would you like to be added:
I hope the author can add the Last (time. Time) time.Time method to the schedule interface,return the last activation time, earlierthan the given time.

You can even extend Last (time. Time, Num int) time. Time and Next (time. Time, Num int) time. Time methods. Num represents the last num times.

example:
schedule, _ = cron.Parse("0 0 0/1 * * ?")
now := time.Now()
schedule.Next(now)
schedule.Last(now)
schedule.Next(now,2)
schedule.Last(now,3)