Support/test unicode/punnycode domains

[gjtorikian]

Should close #480

[codecov-io]

Codecov Report

Merging #486 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #486      +/-   ##
==========================================
+ Coverage   98.27%   98.27%   +<.01%     
==========================================
  Files          27       27              
  Lines        1794     1802       +8     
==========================================
+ Hits         1763     1771       +8     
  Misses         31       31

Impacted Files	Coverage Δ
spec/html-proofer/links_spec.rb	99.24% <100%> (+0.01%)	⬆️

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