rdf2sparql: implement filter function

[mkotoff]

In the automated generated query from ttl we need filter by string, but in the current version it's no possible because it's add variable name in the SPARQL if make filter macro.

Can you please handle it

[VladimirAlexiev]

Ok, it'll look something like this:

<content_id> a s:WebPage;
  puml:filter "?status='published'";
  s:name "(title)";

and will modify the SPARQL query like this:

CONSTRUCT/INSERT {
  ?content_id_URL s:name ?title
} WHERE {
  service <refine> {
    ...
    bind(iri(?content_id) as ?content_id_URL)
    filter(?status='published') ### here
  }
}