Faiss integration

[adibaba]

• Origin: ES memory issues, see issue High memory usage ES on 180 mio index #49
• Created a new branch for integration: https://github.com/dice-group/embeddings.cc/tree/faiss
• Simple example: https://github.com/facebookresearch/faiss/wiki/Getting-started
• Extended example (Dense Passage Retrieval): https://github.com/facebookresearch/DPR/blob/master/dpr/indexer/faiss_indexers.py