warn on reference to missing attribute if attribute-missing is "warn"
warn on reference to missing attribute if attribute-missing is "warn"

- warn on reference to a missing attribute if attribute-missing=warn
- warn on reference to a missing attribute if attribute-missing=drop-line
- remove call to debug when dropping line with reference to a missing attribute
- optimize & cleanup attribute substitution

glamouracademy/asciidoctor,anthonny/asciidoctor,gjtorikian/asciidoctor,benignbala/asciidoctor,gjtorikian/asciidoctor,pepijnve/asciidoctor,glamouracademy/asciidoctor,neontapir/asciidoctor,mjang/mj-asciidoc,yukisan/asciidoctor,cuit-zhaxin/asciidoctor,benignbala/asciidoctor,asciidocfx/asciidoctor,pepijnve/asciidoctor,cuit-zhaxin/asciidoctor,mjang/mj-asciidoc,neontapir/asciidoctor,anthonny/asciidoctor,asciidocfx/asciidoctor,yukisan/asciidoctor