Add support for skipping URL rewrites on certain elements

[Mange]

This PR supersedes #137; it adds extra documentation and integration tests.

Thank you @hasghari for the basis of this feature.

[codecov-io]

Codecov Report

Merging #154 into master will increase coverage by 0.01%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #154      +/-   ##
==========================================
+ Coverage   97.92%   97.94%   +0.01%     
==========================================
  Files          57       57              
  Lines        1884     1896      +12     
==========================================
+ Hits         1845     1857      +12     
  Misses         39       39

Impacted Files	Coverage Δ
lib/roadie/asset_scanner.rb	100% <ø> (ø)	⬆️
spec/lib/roadie/asset_scanner_spec.rb	100% <ø> (ø)	⬆️
spec/integration_spec.rb	100% <100%> (ø)	⬆️
spec/lib/roadie/url_rewriter_spec.rb	100% <100%> (ø)	⬆️
lib/roadie/document.rb	100% <100%> (ø)	⬆️
spec/lib/roadie/document_spec.rb	100% <100%> (ø)	⬆️
lib/roadie/url_rewriter.rb	100% <100%> (ø)	⬆️

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[hasghari]

Awesome, thanks!

[Mange]

Version 3.3.0 was just released with this included.