Coreference resolution and other fixes

[ColdTeapot273K]

1) Implemented a fix for coreference resolution functionality not working properly. The problem involved:

• spaCy 2.1.0+ dependency conflict w/ neuralcoref 4.0 (details: https://github.com/huggingface/neuralcoref/issues/197)
• importing and explicitly loading spaCy language models

Also updated README to provide clearer instructions on how to setup and use coreference resolution functionality.

2) Added instructions on using custom models (loaded from with transformers library), because there's a caveat w/ default model configs

UPD: The build fails because it doesn't have spaCy model installed. We could therefore add it to requirements.txt: en-core-web-sm==2.1.0

UPD#2: Tested again, spacy==2.1.3 works fine so updated requirements and README

[dmmiller612]

Thanks a ton for doing this! It looks like this fails the travis check, since the dockerfile will need to install the english spacy file.

[ColdTeapot273K]

Added a small (11mb) model as a dependency explicitly, now the coreference should work "out of the box", in addition to passing Travis checks.

[dmmiller612]

Thanks a ton!