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
Allow custom override of code env styling

[Delivers #64114122]
Load styles in right order

[Delivers #64114122]
Recover from MessagePack::UnpackError when reading from highlight cache

[Delivers #64492504]
[#64492504]

* 64492504-remove-highlight:
  Recover from MessagePack::UnpackError when reading from highlight cache
  Load styles in right order
  Allow custom override of code env styling
Bump version number
Merge branch 'master' into stable