resolves #56 fix xref to sections
resolves #56 fix xref to sections

- build on work by @davidgamba (see #94)
- retrieve the anchor name from the fragment attribute
- handle inter-document xrefs by consulting the path attribute
- honor the reftext of the destination
- warn if an xref cannot be resolved
- don't inherit when resolving attributes on anchor node

hmflash/asciidoctor-pdf,hmflash/asciidoctor-pdf,DavidGamba/asciidoctor-pdf,DavidGamba/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,Hextremist/asciidoctor-pdf,dsisnero/asciidoctor-pdf,Hextremist/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,theimdal/asciidoctor-pdf,abatalev/asciidoctor-pdf,abatalev/asciidoctor-pdf,theimdal/asciidoctor-pdf,dsisnero/asciidoctor-pdf