fix for outFile option; respect file-extension (#1228)

[lqdchrm]

suggestion for a fix

thx for this awesome tool and keep up the good work 😄

Closes #1228

[codecov-io]

Codecov Report

Merging #1229 into master will decrease coverage by 1.06%.
The diff coverage is 80%.

@@            Coverage Diff             @@
##           master    #1229      +/-   ##
==========================================
- Coverage   88.28%   87.22%   -1.07%     
==========================================
  Files          77       77              
  Lines        4355     4117     -238     
==========================================
- Hits         3845     3591     -254     
- Misses        510      526      +16

Impacted Files	Coverage Δ
src/Bundle.js	98.42% <80%> (-0.77%)	⬇️
src/transforms/htmlnano.js	72.22% <0%> (-27.78%)	⬇️
src/workerfarm/errorUtils.js	64.7% <0%> (-17.65%)	⬇️
src/transforms/posthtml.js	84.37% <0%> (-15.63%)	⬇️
src/visitors/dependencies.js	81.52% <0%> (-13.05%)	⬇️
src/transforms/babel.js	88.55% <0%> (-6.97%)	⬇️
src/assets/HTMLAsset.js	85.14% <0%> (-3.97%)	⬇️
src/visitors/globals.js	92.3% <0%> (-3.85%)	⬇️
src/utils/config.js	86.95% <0%> (-1.45%)	⬇️
src/utils/getTargetEngines.js	96.8% <0%> (-1.07%)	⬇️
... and 19 more

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