Fix relative paths on Windows

[jbsulli]

Stepping through the code, the issue comes from glob returning filepaths with forward slashes while everything else returns Windows-style backslashes. Normally, node is pretty good at handling these differences but someone was trying to just do a quick string replace instead of using path.relative().

[jbsulli]

This is similar to this pull request... just without an added dependency and using Node's path module.

[dancingfrog]

Awesome! At the moment I've had to add additional tasks to my webpack config in order to work around problems with squarespace assemble, which simply won't run successfully on my Windows machine. The fix from this pull request would make a big difference to my work flow!