Add image-based backend #13

add another backend using image instead of sdl2, for people who just want to save plots to image files directly, which requires fewer dependencies and would be compatible with batch processing. There's a branch here which has a new draw_image.rs file, which has some of the stuff stubbed out for this. The commented-out chunks of code are the SDL2 implementations, for ease of reference of what needs to be translated into image commands.