Better cancellation behavior

[nminchow]

Currently, subscription cancellation is instant. We need to do one of three things:

1. communicate that this is the case
2. instead of cancelling the subscription use this flag a. this would entail the pro command being smart enough to check for this case and handle appropriately
3. Use proration. a. We would have to ensure that this isn't abused for features that don't rollback when a subscription cancels. This would probably also entail documentation changes.