Consider which class variables should be made into instance variables

[tleonhardt]

PR #272 brought up the fact that now that users can nest instances of cmd.Cmd2 to support creating sub-menus, we should revisit which class variables should be made into instance variables to prevent potential problems.

[tleonhardt]

@kotfu Do you think that #284 should have closed this? Or do you think we should reopen it and work on it further?