Compiler interpolation
The `Database` class has an `interpolation` attribute which you could set:

``` python
class OracleDatabase(Database):
    interpolation = '%s'
```

Unfortunately there is no support for automatically numbering parameters. Will `%s` work?

Hi, 

I am trying to use peewee with cx_Oracle. So far with limited success. When I issue 

`inst.save()` it compiles an expression with `?`. e.g I have a model : 

```
  class TestDBOz(pw.BaseModel):
          sessionid = pw.IntegerField()
          action = pw.Charfield()

  sid=TestDbOz(sessionid=123, action='Test') 
  sid.save()
```

The above compiles to:

```
  'INSERT INTO "testdboz" ("sessionid", "action") VALUES (?, ?)', [123, Test])'
```

Which sadly, is not accepted by oracle. I was wondering if there is a quick way to replace interpolation with a function that yields a numeric interpolation:

```
 'INSERT INTO "testdboz" ("sessionid", "action") VALUES (:1, :2)', [123, Test])'
```
