Construct a KnowledgeBase from a Wikidata Query

[cthoyt]

This PR is a spin-off from #542 that addresses #346.

This PR adds a utility function for constructing a knowledge base from a SPARQL query to Wikidata (requires pip install wikidata-client) with the following:

from scispacy.linking_utils import KnowledgeBase

# this SPARQL query gets named cats
sparql = """
SELECT ?item ?itemLabel ?itemDescription ?itemAltLabel
WHERE {
    ?item wdt:P31 wd:Q146. # Must be a cat
    SERVICE wikibase:label { bd:serviceParam wikibase:language "[AUTO_LANGUAGE],mul,en". }
}
"""

kb = KnowledgeBase.from_wikidata(sparql, "item")