Is it possible to add custom operator in where method?

Hi!

I use Postgres with jsonb field. I would like to write come expressions like this:

```
select * from table where data::jsonb @> '{"1" : [8]}';
```

I did try to put sting as argument in where method.

```
> Query().from_table('t').where("""data::jsonb @> '{"1" : [8]}""").get_sql()
'SELECT t.* FROM t WHERE ()'
```
