Add more skippable rel types

[gjtorikian]

Should close #481

[codecov-io]

Codecov Report

Merging #484 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #484      +/-   ##
==========================================
+ Coverage   98.26%   98.27%   +<.01%     
==========================================
  Files          27       27              
  Lines        1789     1794       +5     
==========================================
+ Hits         1758     1763       +5     
  Misses         31       31

Impacted Files	Coverage Δ
lib/html-proofer/check/links.rb	98.79% <100%> (+0.01%)	⬆️
lib/html-proofer.rb	100% <100%> (ø)	⬆️
spec/html-proofer/scripts_spec.rb	100% <100%> (ø)	⬆️
spec/html-proofer/favicon_spec.rb	100% <100%> (ø)	⬆️

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