Color and urgency coefficients not working with UDA dates

# The problem
Say I have a date UDA named `when`, configurations like the ones below:
```
urgency.uda.when.today.coefficient=10
color.uda.when.today=rgb045
rule.precedence.color=uda.when,deleted,completed,active,keyword.,tag.,project.,overdue,scheduled,due.today,due,blocked,blocking,recurring,tagged,uda.
```
have no effect, and I expect tasks with the `when` attribute as today to be colored sky blue and have +10 urgency.

Perhaps it doesn't work because the configurations above are meant for UDAs with the `uda..values` config set, but it would be nice to have the ability to customize UDA dates in this fashion, as it works perfectly fine with the `due` attribute.