add staging and cleanup operations before/after all config checks are evaluated

[kristovatlas]

Staging operation examples:

• Check if user is in sudoers (sudo -vn 2>&1 >/dev/null | grep -c "a password is required"):
  • If the user is in suduoers, this will return '1'
  • If the user is not in sudoers, this will return '0'
• Explain what sudo does and ask if user would like to grant this privilege. If yes, explain that the app is about to prompt the user for his OS X user's password. Also, explain to him that he won't be able to see "*"'s as they type in the password -- one user who had never used sudo before found this confusing.

Cleanup operations

• Clear sudo state (sudo -k)