resolves #732 list preface with title in table of contents (PR #743)
resolves #732 list preface with title in table of contents (PR #743)

- in the case the preface is defined using a preamble block, and the
  preface has a title, promote it to a special section so it gets listed
  in the table of contents
Hextremist/asciidoctor-pdf,Hextremist/asciidoctor-pdf,asciidoctor/asciidoctor-pdf,dsisnero/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,dsisnero/asciidoctor-pdf,mojavelinux/asciidoctor-pdf,asciidoctor/asciidoctor-pdf