fix: fix path handling error on windows

[tkak]

On windows, template_dir resource is not working correctly when it handles subdirectories like

Error: open output/dir\foo.txt: The system cannot find the path specified.

I replaced path package with path/filepath package to fix the issue.

References: https://stackoverflow.com/questions/39181790/whats-the-difference-between-path-path-filepath-packages-in-go

This PR fixes #73