Feature. Provide sets of meta keys for inclusion

Feature. Provide sets of meta keys for inclusion

Add further lists of meta keys to include (*hash_meta_keys*
and *list_hash_meta_keys*), add them to the *list_meta* map in
subroutine *htmlize* and provide them to *pagestate* *meta*.

This makes available, e.g. the *reference* attribute in page
templates.

The commit require some explanation.

Until now, meta attributes were available as

 * Bolean meta (*bool_meta*)
 * Scalar meta (*scalar_meta*)
 * List meta (*list_meta*)

All meta attributes in documents are provided as page template
variables with a 'pandoc_'-prefix. On top of that, attributes listed
in *scalar_meta_keys* are made available as regular template
variables, without prefix.

Lists are problematic. Pure lists are pushed with 'pandoc_' prefix,
but they can not be used. The alternative would be via loops (see
<https://metacpan.org/pod/HTML::Template#TMPL_LOOP>), but inside loops
one need to call for a variable and the pushed attribute does not
provide one.

This commit properly makes available the *reference* attribute, which
is of type list of associated list. This construct actually fully
natural fulfil the limitations of lists. Template loops generate the
items, whose values are available with keys as variable names:

    ---
    references:
     - id: refone
       title: Ref-MetaInlines-in-MetaList
     - id: reftwo
       title: Ref-MetaInlines-in-MetaList
    ...

Which is called like this:

    <TMPL_IF REFERENCES>
      <TMPL_LOOP REFERENCES>
         <TMPL_VAR ID>
         <TMPL_VAR TITLE>
      </TMPL_LOOP>
    </TMPL_IF>

How is hacky. The items in *hash_meta_keys* and *list_hash_meta_keys*
are added to *list_meta_keys* and undergo the list processing. The
list processing contradictory adopted better for *list_hash_meta_keys*
than for *list_meta_keys*.