Squash single selected image

[takluyver]

Follow up from #165 - we now have a script that can squash all the logo images. But we're probably often going to ask people to squash the logo they're adding. They have to wait for pngcrush to brute-force all the already squashed images, and if they have a different version, it might modify the images.

So it would be great if we had the option to run scripts/squash-images.sh assets/newlogo.png to resize and compress a single image.