EPUB writer:  Improved metadata handling.

* Metadata may now be included in YAML blocks in a markdown document.
  For example,

        ---
        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
        ...

* Metadata may still be provided using `--epub-metadata`; it will
  be merged with the metadata in YAML blocks.
* meta tags are used instead of opf attributes for EPUB3.