Add an ability to configure user.name and user.email

[HiromuHota]

If user.name and user.email are already configured, they are picked up and will be used when making a commit. But if not, they are automatically created based on system username and hostname like root <root@d9a7c35431d9>, which is not always preferable. I'd like to configure user.name and user.email on SpoonGit.