Add Wikidata as a possible EntityLinker?

Hello,

SciSpacy is awesome, I have been using it from time to time. I use [Wikidata](https://www.wikidata.org/wiki/Wikidata:Main_Page) as a single go-to ontology, and it would be awesome to have Wikidata integration in the EntityLinker in sciSpacy. It would be super useful to have wikidata IDs as long as the UMLS IDs, for example.

I do not know the implementation details, so I do not know how much of a challenge it would be. Wikidata has +100M concepts, but only a fraction are relevant. It would already be super cool to have the 6.2M that are linked to English Wikipedia.

If there is something in the side of Wikidata that could be useful (e.g. making some subset of terms), just let me know and I would love to help!

Best,
Tiago