Integrate with PyTextRank for entity linking

[ceteri]

Use a graph defined by kglab to enhance entity extraction in PyTextRank

• add support specifically for entity linking
• coordinate with @louisguitton
• submit kglab to spaCy Universe

[ceteri]

While we're working on TextRank-meets-KG, it's probably a good time to consider:

• fact checking with triples in DeFactoNLP
• evidence retrieval with https://github.com/DeFacto/EvidenceRetrieval-ClaimClassification

See also: fact extraction in https://github.com/DeFacto/fever-naacl-2018