Drill difficulties levels

Kanji drills are sometimes too easy, when you start to know a list. You can have the right answer, not because you know it, but because you know that the others choice are not the good one.
For the kanjis drill, being able to have a level of difficulties may be a possible option to solve that.

Kanjis:
- Low: pick bad answers at random in the list (like it work now)
- Medium: pick up to 2 kanjis with at least one radical in common with the right answer/question.
- High: pick up 6 kanjis with radicals in common with the right answer (if possible).

Kana:
- Low: all bas answers takes at random
- Medium: 2 bad answers with at least one Latin letter in common.
- High: All answers are closest possible to the right answer.