fromFile and toFile don't respect images in sketch file

[connorwyatt]

Hi, when I'm loading a file to modify and then saving again, the images aren't present in the zip. I realise that you may not want it to be default behaviour if you're worried about large files in memory, but the option to have them loaded would be very useful.

An alternative I can see is to allow a --merge option to the toFile method that would merge the keep the images folder from the file that's already on disk if it exists. (Although this seems fraught with issues)