[TW-1503] Make due date urgency term configurable

_Michael Ekstrand on 2015-01-07T21:19:35Z says:_

Right now, the due date term in the urgency computation is hardcoded to interpolate in the range \[0.2,1.0], based on a 3-week window. This has the effect that setting a due date always bumps the urgency of a task over tasks without due dates.

For my workflow, I would like to be able to have tasks without due dates be sorted before tasks with due dates a ways out. I can do this in a crude fashion with 'waiting' times, but that entirely suppresses the tasks rather than just pushing them down the list.

Being able to adjust the window (e.g. `urgency.due.floor=0`), or have even finer-grained control over the due date curve would fix this (pass days overdue through a customizable sigmoid function?).

This is somewhat related to TW-1305, but addresses a different concern.