Add MWSC
Looks good to me
Adding the [Modified Winograd Schema Challenge](https://github.com/salesforce/decaNLP/blob/master/local_data/schema.txt) dataset which formed part of the [decaNLP](http://decanlp.com/) benchmark. Not sure how much use people would find for it it outside of the benchmark, but it is general purpose.

Code is heavily borrowed from the [decaNLP repo](https://github.com/salesforce/decaNLP/blob/1e9605f246b9e05199b28bde2a2093bc49feeeaa/text/torchtext/datasets/generic.py#L773-L877).

There's a few (possibly overly opinionated) design choices I made:

- I used the train/test/dev split [buried in the decaNLP code](https://github.com/salesforce/decaNLP/blob/1e9605f246b9e05199b28bde2a2093bc49feeeaa/text/torchtext/datasets/generic.py#L852-L855)
- I split out each example into the 2 alternatives. Originally the data uses the format:
    ```
    The city councilmen refused the demonstrators a permit because they [feared/advocated] violence. 
    Who [feared/advocated] violence? 
    councilmen/demonstrators
    ```
    I split into the 2 variants:
    ```
    The city councilmen refused the demonstrators a permit because they feared violence. 
    Who feared violence? 
    councilmen/demonstrators
    
    The city councilmen refused the demonstrators a permit because they advocated violence. 
    Who advocated violence? 
    councilmen/demonstrators
    ```
    I can't see any use for having the options combined into a single example (splitting them is [the way decaNLP processes](https://github.com/salesforce/decaNLP/blob/1e9605f246b9e05199b28bde2a2093bc49feeeaa/text/torchtext/datasets/generic.py#L846-L850)) them. You can't train on both versions with them combined, and splitting the examples later would be a pain to do. I think [winogrande.py](https://github.com/huggingface/nlp/blob/master/datasets/winogrande/winogrande.py) presents the data in this way?

- I've not used the decaNLP framing (appending the options to the question e.g. `Who feared violence? 
 -- councilmen or demonstrators?`) but left it more generic by adding the options as a new key: `"options":["councilmen","demonstrators"]` This should be an easy thing to change using `map` if needed by a specific application.

Dataset is working as-is but if anyone has any thoughts/preferences on the design decisions here I'm definitely open to different choices.