os specific logic should be extracted into its own package

[whyrusleeping]

Anything os specific (like HOME, and executable names) should get its own subpackage so we can keep that logic together and well tested

[godcong]

Does it mean something like these if runtime.GOOS == "windows" ?
Need to be changed to // +build windows flag?

[gammazero]

Home directory expansion and home directory determination are now done using the github.com/mitchellh/go-homedir package.