When someone configures their `nextercism` CLI for the first time, `workspace_dir` will be blank in the config. We need to set a sensible default based on their operating system. - Mac: `~/Exercism` - Linux: `~/exercism` - Windows: `%HOMEDRIVE%%HOMEPATH%\Exercism` @rpottsoh do you have any suggestions for what a good default Exercism directory would be for Windows?