feat: fill in git options user.email and user.name if not set

[wpyoga]

I'm proposing this patch to fix #48

Rationale:

• sometimes user.email and user.name are not configured globally
• we don't want to override global settings, so:
  • if user.email and user.name are configured globally, use those
  • otherwise, ask the user and set those options locally (for the current repo, which will be the one pushed to GitHub/GitLab)