v4.0.0: Simpler installation and integration with SpaCy 2.1+, simple domain-knowledge integration

• Now on pip: pip install neuralcoref.
• Simpler installation which should hopefully be easier to maintain compatible with SpaCy from now on.
• Only use one model and only store/download/cache the coreference resolution model and not the SpaCy models anymore: 10x smaller to download and store
• Compatible w. SpaCy 2.1+: we probably won't seek backward compatibility for older versions of SpaCy since the dependency structure has been cleaned up a lot in SpaCy 2.1 (impressive job by Inès and Matthew! Read about it here)
• New feature to incorporate domain knowledge: conversion dictionary (see details in the readme).