Fix optimizeImages configuration

[andysaman]

Resolved Issue #110:

• This adds the ability to skip optimization during production builds if desired.

[codecov]

Codecov Report

Merging #115 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master    #115   +/-   ##
======================================
  Coverage    90.9%   90.9%           
======================================
  Files          13      13           
  Lines         176     176           
  Branches       41      41           
======================================
  Hits          160     160           
  Misses         13      13           
  Partials        3       3

Impacted Files	Coverage Δ
lib/index.js	94.73% <ø> (ø)	⬆️

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[cyrilwanner]

Initially, this config value was only meant to disable the warning, but it makes total sense to actually disable the optimization.
Thank you very much for your contribution! This fix will be published to npm in a few minutes.