Add ability for cost of delay to increase or decrease by a date

[sfradkin]

Sometimes for a story the cost of delay significantly increases or decreases at a particular time. Add the ability to define a max or min cost of delay value. The fcli backlog score command will use the due date of the story plus the min or max cost of delay value defined to calculate what the cost of delay is at the current point in time, then recalculate the VFR value of the story accordingly.