Add yaml include directive support

[Levi-Armstrong]

This adds a utility function for loading yaml files which now support the use !include directive to allow separating a yaml file into multiple files where relative paths are supported.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 89.23%. Comparing base (92fa862) to head (8de0790).
Report is 2 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1091      +/-   ##
==========================================
- Coverage   89.31%   89.23%   -0.09%     
==========================================
  Files         288      289       +1     
  Lines       16320    16380      +60     
==========================================
+ Hits        14577    14616      +39     
- Misses       1743     1764      +21     

see 11 files with indirect coverage changes