Moved the region's add method to the general container abstraction since such
functionality is useful for other nodes such as table cells provided by the
table parser.
1 = Level 1 =
2 Text
3 == Level 2 Heading ==
4 Text
5 Not == a heading ==
6 == Not a heading == either
7 = Mismatched heading ==
8 == Another mismatched heading =
9 === Heading __Underlined__ ===