**Added**
* Arkanoid and Pingpong:
* The serving position and direction of the ball can be decided
* Add difficulties for different mechanisms
* Add ball slicing mechanism
* Arkanoid: Add hard bricks
* Pingpong: Add blocker
**Changed**
* Update the python from 3.5 to 3.6: For the `auto()` of the custom `Enum`
* Optimize the output of the error message
* Refactor the game classes: Extract the drawing and recording functions
* Add prefix to the filename of the record files
* Physics: Optimize the ball bouncing algorithm