"SKIP LOCKED" support (request)

I have noticed that sequelize does not support "SKIP LOCKED" selects.

A query example with "FOR UPDATE" and "SKIP LOCKED" would be
```
SELECT "id" FROM "Customers" WHERE "Customers"."AccountBalance" > 5 LIMIT 3 FOR UPDATE SKIP LOCKED;
```

"SKIP LOCKED" is useful for job queues.

https://www.postgresql.org/docs/9.5/static/sql-select.html#SQL-FOR-UPDATE-SHARE