Add ZEST: ZEroShot learning from Task descriptions
Looks good to me, we can ping the authors for more info later. And yes apply `other-task` labels liberally, we can sort them out later :) 

Looks ready to merge when you're ready @joeddav 
Adds the ZEST dataset on zero-shot learning from task descriptions from AI2.

- Webpage: https://allenai.org/data/zest
- Paper: https://arxiv.org/abs/2011.08115

The nature of this dataset made the supported task tags tricky if you wouldn't mind giving any feedback @yjernite. Also let me know if you think we should have a `other-task-generalization` or something like that...