Implement includable code sections

Code files can now be marked up into sections; individual sections can then be included into a book.
Merge branch 'includable_code_sections' of https://github.com/skmetz/polytexnic into skmetz-includable_code_sections
Make anal changes
Merge branch 'skmetz-includable_code_sections'

* skmetz-includable_code_sections:
  Make anal changes
  Implement includable code sections
Merge branch 'master' into 65605970-force-png