[Feature] Allow to include _some_ of the historic contents

[erhant]

Our SDK is capable of storing content ids in a history, and make sure if a content exists in history it is not returned, and a new one is given instead. This makes sense for a number of use-cases, but a relaxed version of this should also be possible.

In my case, the user is solving a set of questions; say A B C D. The following happens:

• She solves A, and does not want to see this question again in a batch. The SDK moves away from questions like A as well.
• She solves B but signals that she would like to see it again in a future batch. The SDK stays around similar questions for personalization.
• She cant solve C and definitely wants to try more related questions.
• She didn't attempt D at all, so it should be returned again in a future batch.

The described scenario is similar to flash-card training programs.