Fix: Folders no longer created

jszip has been changed (https://github.com/Stuk/jszip/issues/130) to not automatically create folders anymore in favour of allowing filenames with a slash.
This fixes it by using jszip's folder function.
In the previous line we already translate backwards slashes to forwards slashes already.
I could merge them together using path.sep if file.relative is always returning the path using platform specific separators.