Flip image horizontally
Sometimes I need to have image directed both left and right. So I have to draw 2 images:

![image](https://user-images.githubusercontent.com/21060932/67076735-6f86bb80-f196-11e9-8d5a-93805f93e196.png)

It would save lots of time to use additional argument like `flip_horizontally=True` to flip image:
`pyxel.blt(x, y, img, u, v, w, h, colkey, flip_horizontally)`

P.S. Also `flip_vertically` and `rotate` would be great, but `flip_horizontally` is most wanted feature.