Blinking those horizontal rows to be cleared

1. Modify your previous TETRIS program. Since you know which row(s) to print out a mark "->" aside, we want to show the game in a more advanced way -- blink the row before we clear it.
2. To make it look like "blinking", actually we simply turn all the cells in the horizontal rows to become WHITE (color 3), and then turn them to be BLACK (color 0) again.