Allow querying chainfind with sql conditions

Enables:
```js
Person.find({ age: 18 }).where("LOWER(surname) LIKE ?", ['dea%']).all( ... );
Person.find({ age: 18 }).find("LOWER(surname) LIKE ?", ['dea%']).all( ... );
Person.find({ age: 18, __sql: [["LOWER(surname) LIKE ?", ['dea%']]] }, function (err, items) { ... });
```
but I think we should only document the first version.

I named the key `__sql` to avoid any collisions with user specified model properties.

This addresses #225 and depends on [https://github.com/dresende/node-sql-query/pull/16](https://github.com/dresende/node-sql-query/pull/16) (I think tests will fail until that version of sql query is published).