run julia demos to generate assets before passing into Literate

[johnnychen94]

closes #5

#7 introduces a postprocess callback function, this PR runs each julia demo files to generate potential assets, so that following usage becomes possible (and it's really useful):

using TestImages, FileIO

img = testimage("cameraman")
save("assets/cameraman.png", img) # hide

# ![cover](assets/cameraman.png)