Allow to specify the name of Primary Key

I've a database schema (MySQL in my case) which has no key named `id` but it uses `idcustomer` as primary key. In this way, `node-sql-fixtures` can't work. 

```
 ER_BAD_FIELD_ERROR: Unknown column 'id' in 'where clause'
```


It should be nice to have the option to specify the primary key of table to avoid error.