Support title pages

[cxw42]

Various options:

• Wren
• Perhaps using template-glib, which already has Vala bindings.
• Does rst, adoc, pod6, or another format provide enough flexibility to represent title pages? If so, maybe write the corresponding reader first and use that reader to generate the title-page elements.
• YAML frontmatter in Markdown? https://github.com/rainwoodman/libyaml-glib/ , https://github.com/jimmuhk/glib-yaml/
• Data in special block
• Additional properties to hold title, subtitle, author

I would like to avoid having multiple sets of properties: (size, alignment, justification) x (title, subtitle, header, footer, heading, copy) is way too much for a flat list of properties.