def todo_with_substring(substring)
      issue_todo = @remaining_todos.detect{ |t| !t.content.index(substring).nil? }
      issue_todo ||= @completed_todos.detect { |t| !t.content.index(substring).nil? }
    end
def todo_with_substring(substring)
      issue_todo = @remaining_todos.detect{ |t| !t.content.index(substring).nil? }
      issue_todo ||= @completed_todos.detect { |t| !t.content.index(substring).nil? }
    end
searches the remaining and completed todos for the first todo with the substring in its content

 @param [String] substring substring to look for
 @return [Logan::Todo] the matched todo, or nil if there wasn't one