Metadata syntax (was Universal syntax for Markdown)

[John MacFarlane]

Two postscripts:

1. Multiline strings are also possible:

    <!--@
    abstract   = m[[
      Here's my abstract.  It has several
      lines and an ordered list.

      * first item.
      * second item.
      ]]
    -->

2. lua is about the same size as libyaml and extremely portable.