Add support for nested code fences

[Delivers #63658240]
Issue pull request for branch 63658240-nested-fences

[Delivers #63658240](https://www.pivotaltracker.com/story/show/63658240)
[Delivers #63261490](https://www.pivotaltracker.com/story/show/63261490)
[Delivers #63249876](https://www.pivotaltracker.com/story/show/63249876)
Merge pull request #98 from softcover/63658240-nested-fences

63658240 nested fences