• Make Option.as_list() use the current value after assignments to or from
  None, so append() no longer discards values added to a valueless option
  and converting an option to None no longer raises an AttributeError, PR #179
• Detach every child block when clearing a document or section, so removed
  sections and options can be attached elsewhere without stale parent links
  (follow-up to PR #43), PR #180