export Core.JULIA_IMAGE_FILE

As discussed with @JeffBezanson and @jakebolewski, this adds `Core.JULIA_IMAGE_FILE` so that you have access in Julia to the image filename.

e.g. this is useful in pyjulia, or any other program that wants to use the `jl_init_with_image`, so that it can get the image-file path from Julia.
This might need to be done differently since `jl_set_const` will not overwrite existing values.