feat: add option keepUselessDefs

[gregberge]

Fix #36

[codecov]

Codecov Report

Merging #42 into master will increase coverage by 0.23%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master      #42      +/-   ##
==========================================
+ Coverage   45.33%   45.56%   +0.23%     
==========================================
  Files          22       22              
  Lines         236      237       +1     
  Branches       54       55       +1     
==========================================
+ Hits          107      108       +1     
  Misses        100      100              
  Partials       29       29

Impacted Files	Coverage Δ
src/cli/index.js	0% <ø> (ø)	⬆️
src/configToOptions.js	87.5% <100%> (+0.54%)	⬆️

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