OR operator in multiple where conditions

Currently is not possible to specify the operator kind between multiple WHERE conditions, AND condition is assumed:
```sql
WHERE ((((t2."kind" = ?) AND (t2."value" <= ?)) *OR* ((t2."kind" = ?) AND (t2."value" <= ?))) AND (t1."state" = ?))
```
Now is possible to specify the operator for each WHERE condition, using the op=(peewee.OP_OR, peewee.OP_AND)

```python
q = Foo.select()

for i, val in conditions:
     q = q.where((Foo.i == val), op=peewee.OP_OR)
```