Do not apply windows_safe_path to all arguments

This PR changes the way we apply `windows_safe_path`. Instead of blindly applying it everywhere, we selectively apply it based only on **our** inputs. Now it's up to the user to implement their paths correctly.

- Fixes #255 

/cc @btm @smurawski @adamedx 
I don't know enough about how `Mixlib::ShellOut` works to know if this change is required... /cc @danielsdeleo