[New] prop-types: add skipUnexported option

[GertSallaerts]

Implements #494

[codecov]

Codecov Report

❌ Patch coverage is 85.71429% with 8 lines in your changes missing coverage. Please review.
✅ Project coverage is 97.64%. Comparing base (012ec39) to head (3c09cb9).

Files with missing lines	Patch %	Lines
lib/rules/prop-types.js	85.71%	8 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #3999      +/-   ##
==========================================
+ Coverage   97.58%   97.64%   +0.06%     
==========================================
  Files         137      137              
  Lines       10182    10238      +56     
  Branches     3795     3824      +29     
==========================================
+ Hits         9936     9997      +61     
+ Misses        246      241       -5     

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