Add getValOnce functionality in processing rules

[#1]

In processing rules, I woud like to see an option when setting up a condition to preform the action if the value of a variable has changed since the last time it was recorded. So for example only copy the value of prop1 to eVar1 if the value of prop 1 has changed since the last time it was recorded during that visit. I understand that processing rules cannot read cookies, so it would need to go back and look at the previous hits in that visit to check for the value. This basically will give us the functionality of the getValOnce plugin within processing rules.