Add a config for ESM compatibility

[hmenzagh]

Simple PR to add a config.esm property to load models on the .default in the case that ESM is used

[codecov]

Codecov Report

Merging #253 into master will increase coverage by 0.01%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #253      +/-   ##
==========================================
+ Coverage   81.49%   81.51%   +0.01%     
==========================================
  Files          24       24              
  Lines        2804     2807       +3     
==========================================
+ Hits         2285     2288       +3     
  Misses        519      519              

Impacted Files	Coverage Δ
config.js	100.00% <100.00%> (ø)
rest-hapi.js	90.81% <100.00%> (ø)
utilities/joi-mongoose-helper.js	89.20% <100.00%> (ø)
utilities/model-generator.js	82.00% <100.00%> (+0.36%)	⬆️
utilities/rest-helper-factory.js	80.39% <100.00%> (+0.03%)	⬆️

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