Add use_value option to KPI component

[Donnype]

Also see: #4 (comment):

Another feature you could consider adding for this use case is: a flag called use_value with default False. If True, we display the absolute difference instead of relative. This does require changing the template to handle this as well though.

This makes more sense for the KPI component when the target is (near) 0 or the absolute difference is just more interesting.