manually forming paths for etree consumption for Windows compatibility

It seems like the etree library expects paths to be formed with forward
slashes (because why wouldn't it?). Unfortunately on Windows systems,
path.join uses backslashes, causing a lot of "cannot call asText() on
undefined" errors.

If you'd rather resolve this another way no hard feelings - As long as a fix for windows platforms eventually ends up in npm. This library is quite practical for generating xlsx files. Thanks!