Provide custom encryption Salt and Rounds in version 2.x

Before version 2 i was doing this:

```
opts.EncryptionKey = new SymmetricKey("KeySeed", "SaltText", rounds);
```

Now a custom Salt and Rounds cannot be provided:

```
var config = new DatabaseConfiguration
{
    EncryptionKey = new EncryptionKey("KeySeed") // using default salt and rounds
};
```

Will that functionality be provided again?
