stb_image_ffi 1.0.2

Very naive bindings for stb_image.h. Implements only stbi_set_flip_vertically_on_load and stb_load since these are the 2 functions I need to load an image as a texture in OpenGL. The reason I made this instead of using stb-image is because stb-image didn't implement stbi_set_flip_vertically_on_load, as far as I can tell, which is useful/necessary when wanting to load image data in a format immediately consumable by OpenGL (see Example in README).