Allow skipping the href requirement

[gjtorikian]

Closes #474.

[codecov-io]

Codecov Report

Merging #475 into master will increase coverage by <.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #475      +/-   ##
==========================================
+ Coverage   98.25%   98.26%   +<.01%     
==========================================
  Files          27       27              
  Lines        1776     1786      +10     
==========================================
+ Hits         1745     1755      +10     
  Misses         31       31

Impacted Files	Coverage Δ
lib/html-proofer.rb	100% <ø> (ø)	⬆️
lib/html-proofer/configuration.rb	76.92% <ø> (ø)	⬆️
lib/html-proofer/check/links.rb	98.78% <100%> (+0.01%)	⬆️
lib/html-proofer/element.rb	97.56% <100%> (+0.04%)	⬆️
spec/html-proofer/links_spec.rb	99.23% <100%> (+0.01%)	⬆️

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