Add support for the data and data-about elements in the plugin

[ColinMaudry]

The data and data-about elements have semantics that are very close to what the RDF model enables:
http://docs.oasis-open.org/dita/v1.2/os/spec/langref/data.html#data
http://docs.oasis-open.org/dita/v1.2/os/spec/langref/data-about.html#data-about

Why not extracting them as rdf:Statement?

Ideal example:

DITA

<data name="subject" href="http://dbpedia.org/resource/Mexico"/>
<data-about href="http://dbpedia.org/resource/Mexico">
    <data name="population" value="115296767" datatype="http://www.w3.org/2001/XMLSchema#integer"/>
</data-about>

RDF (Turtle syntax)

<http://data.example.com/resources/map/en-us/d5e5/data:1;5:48>
    a rdf:Statement ;
    rdf:subject <http://data.example.com/resources/map/en-us/d5e5> ;
    rdf:predicate <http://data.example.com/resources/predicate/subject> ;
    rdf:object <http://dbpedia.org/resource/Mexico> .
<http://dbpedia.org/resource/Mexico>
    <http://data.example.com/resources/predicate/population> "115296767"^^xsd:integer .

Now, it's probable user will nest elements in various ways... I'm not sure yet how to handle that.

If the <data> element is specialized then the name of the element is used instead of @name.