Chatty-goose

Breaking changes

Renamed several classes to follow Python conventions / be more consistent

`chatty_goose.agents.cqragent` -> `chatty_goose.agents.chat`
`HQE` -> `Hqe`
`T5_NTR` -> `Ntr`
`HQESettings` -> `HqeSettings`
`T5Settings` -> `NtrSettings`
`CQRType` -> `CqrType`
`CQRSettings` -> `CqrSettings`
`CQR` -> `ConversationalQueryRewriter`

BREAKING CHANGES
- Integrate ParlAI Facebook Messenger example for a demo by edwinzhng
- Integrate Pyserini/Pygaggle for a reference implementation of multi-stage passage retrieval by edwinzhng
- Add replication log for TREC CAsT 2019 conversational passage retrieval task by edwinzhng