Add levels to the game

[aydin41k]

1. Remove scoring
2. Add in levels
   2.1. Start from level 10 (i.e., the result should not exceed 10 - currently the limit variable)
   2.2. Every time two right answers are given in a row, the level increases by 1
   2.3. Every time two wrong answers are given in a row, the level decreases by 1
   2.4. A right answer followed by a wrong answer, or a wrong answer followed by a right answer does not change the level

This way, the child can compete with himself/herself - they can find their level and work on improving it.

PR's are welcome. Read readme.md for more info.

[nijek]

Hello, I'd like to do this, can you assign me?