test: if $config.json missing, load $config.yaml

[msimerson]

• additional test for 'missing json loads yaml'
• modify get_path_to_config_dir regex to also match windows paths (\ vs / path separator)
• add tests for get_path_to_config_dir
• configs w/o .ext or declared type default to flat
• add test for !filename overloads

[codecov]

Codecov Report

Merging #28 into master will increase coverage by 1.26%.
The diff coverage is 93.75%.

@@            Coverage Diff             @@
##           master      #28      +/-   ##
==========================================
+ Coverage   79.26%   80.53%   +1.26%     
==========================================
  Files           7        7              
  Lines         410      411       +1     
  Branches      115      118       +3     
==========================================
+ Hits          325      331       +6     
+ Misses         85       80       -5

Impacted Files	Coverage Δ
configfile.js	70.09% <93.75%> (+2.61%)	⬆️

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[msimerson]

Adds another test for json/yaml Overrides:

✔ jsonOverrides - no override for smtpgreeting
Overriding file smtpgreeting with config from /Users/matt/git/haraka-config/test/config/main.json
{ '!smtpgreeting': [ 'this is line one', 'this is line two' ] }
✔ jsonOverrides - with smtpgreeting override