add wikisql
That's great work @ghomasHudson !
Adding the [WikiSQL](https://github.com/salesforce/WikiSQL) dataset.

Interesting things to note:
- Have copied the function (`_convert_to_human_readable`) which converts the SQL query to a human-readable (string) format as this is what most people will want when actually using this dataset for NLP applications.
- `conds` was originally a tuple but is converted to a dictionary to support differing types.

Would be nice to add the logical_form metrics too at some point.