Rename files under subdirectories to flat names
Original file names:
./img/icon/ic_a.png
./img/icon/ic_b.png
./img/map/map_a.png
./img/map/map_b.png
./img/map/asia/korea.png
./img/map/asia/japan.png
Run flat.py:
python flat.py img
New file names:
./img__icon__ic_a.png
./img__icon__ic_b.png
./img__map__map_a.png
./img__map__map_b.png
./img__map__asia__korea.png
./img__map__asia__japan.png