use os/user to determine home directory on Windows

As I reported in #140, `homePath()` is not perfect for Windows (+MSYS).
And old issue #7 and 2e314c66c2c084dc8a110119cccea40f04077c11 had solved it by using `os/user` package.
But those changes are lost imperceptibly.

This pull request uses `os/user` again to determine homePath.