use path.extname instead of regex comparisons

[ghost]

fixes #31

[msimerson]

LGTM

[codecov]

Codecov Report

Merging #32 into master will increase coverage by 0.26%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #32      +/-   ##
==========================================
+ Coverage   81.27%   81.54%   +0.26%     
==========================================
  Files           8        8              
  Lines         422      428       +6     
  Branches      123      124       +1     
==========================================
+ Hits          343      349       +6     
  Misses         79       79

Impacted Files	Coverage Δ
config.js	90.47% <100%> (+0.73%)	⬆️

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