Empty assets directory after install_pyxel_examples
Downloaded the examples using install_pyxel_examples command, the python files are there, but assets dir is empty, so you need to download them from github.

```
Traceback (most recent call last):
  File "02_jump_game.py", line 159, in <module>
    App()
  File "02_jump_game.py", line 9, in __init__
    pyxel.image(0).load(0, 0, 'assets/jump_game_160x120.png')
  File "C:\Users\golebiewski\AppData\Local\Programs\Python\Python36-32\lib\site-packages\pyxel\image.py", line 46, in load
    pil_image = PIL.Image.open(filename).convert('RGB')
  File "C:\Users\golebiewski\AppData\Local\Programs\Python\Python36-32\lib\site-packages\PIL\Image.py", line 2580, in open
    fp = builtins.open(filename, "rb")
FileNotFoundError: [Errno 2] No such file or directory: 'assets/jump_game_160x120.png'
```
