Provide support to change accumulated cost

[fisadev]

The original Problem.cost() on aima-python receives the state, the actions, and the total cost, and expects to a new total cost to be returned. On simpleai we have changed that, to accept only the states and the action, and expect just the cost of that action to be returned. But this doesn't works for problem with actions that alter the costs of the previously taken actions.

[fisadev]

I have the patch to the code, but need to update all the docs. I will do that on the next few days.