Qualifier support

[sileix]

Qualifier is very common in Wikidata questions. In order to support question answering over Wikidata, we need to support it. Basically, it should be something similar to filter on compound thingtalk type. For example, if we want to have "all president of US before 1900":

[id of presidents filter start_date <= new Date(1900) as presidents] of @org.wikidata.country();