adding TabFact dataset
FYI you guys are on GitHub's homepage 😍

<img width="1589" alt="Screenshot 2020-12-09 at 12 34 28" src="https://user-images.githubusercontent.com/326577/101624883-a0ecc700-39e8-11eb-8a97-11af0d036536.png">
 Yeayy 😍 🔥
Adding TabFact: A Large-scale Dataset for Table-based Fact Verification.

https://github.com/wenhuchen/Table-Fact-Checking

- The tables are stored as individual csv files, so need to download 16,573 🤯  csv files. As a result the `datasets_infos.json` file is huge (6.62 MB).
- Original dataset has nested structure where, where table is one example and each table has multiple statements,
flattening the structure here so that each statement is one example.