Use protocol-relative URL for mathjax.

See jgm/pandoc-templates#67.
EPUB, EPUB3 templates:  Handle structured metadata on titlepage.

Previously we just expected 'title', 'subtitle', 'author', 'date'.
Now we still support those, but also support the format recommended
for epub metadata in the pandoc README:

    ---
    title:
    - type: main
      text: My Book
    - type: subtitle
      text: An investigation of metadata
    creator:
    - role: author
      text: John Smith
    - role: editor
      text: Sarah Jones
    identifier:
    - scheme: DOI
      text: doi:10.234234.234/33
    publisher:  My Press
    rights:  (c) 2007 John Smith, CC BY-NC
    ...