Added facility to embed metadata for the DOM filter.

Added facility to embed metadata for the DOM filter.

Added a facility to embed metadata for records generated by the DOM XML
filter. The meta data step is triggered by a 'meta' section in the
retrieve section of the dom config, e.g.
  <retrieve name="myelementset">
    <xslt stylesheet="my-usual.xsl"/>
    <meta/>
  </retrieve>
The meta section substitutes metadata for all elements called 'meta' in
namespace http://indexdata.com/zebra-2.0 . The meta element must have
exactly one attribute, element_set_name, which specifies the special
element set name which is equivalent to the suffix to zebra:: for
non-embedded special retrievals. This allows snippets, facets etc to be
included.