Make attribute list parsing more lenient

[MasterOdin]

Splitting this off from #212

Asciidoctor is more lenient in how it parses attribute lists, making usage of double quotes not strictly necessary in most cases. This is a huge convenience to end users, and simplifies usage tremendously. It would be good to carry this behavior over to asciidoc.py. This test suite from asciidoctor should be migrated into asciidoc.py to TDD the implementation and keep the two inline on this feature.