Implement includable code sections

Code files can now be marked up into sections; individual sections can then be included into a book.
Convert gif filenames to png

[Delivers #65605970]
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
Get chapter label working

[#65817002]
Refactor chapter/section label

[#65817002]
Get toc label working

[Delivers #65817002]
Get figure/table working

[Delivers #65817024]
Get Unicode working in listings and boxes

[#65817046]
Chapter linking, etc., working

[#65817046]
Get linking working for everything

[Delivers #65817046]
[#65817046]

* 65817046-box-listing:
  Get linking working for everything
  Chapter linking, etc., working
  Get Unicode working in listings and boxes
  Get figure/table working
  Get toc label working
  Refactor chapter/section label
  Get chapter label working
  Convert gif filenames to png
Bump version number
Merge branch 'master' into stable