[WIP] Add multi woz
This PR adds version 2.2 of the Multi-domain Wizard of OZ dataset: https://github.com/budzianowski/multiwoz/tree/master/data/MultiWOZ_2.2

It was a pretty big chunk of work to figure out the structure, so I stil have tol add the description to the README.md

On the plus side the structure is broadly similar to that of the Google Schema Guided dialogue [dataset](https://github.com/google-research-datasets/dstc8-schema-guided-dialogue), so will take care of that one next.