Renamed 'focus' config option to 'focused'

The name of the `focus` config option is a source of confusion. I keep getting it wrong in my code and noticed that there are also several errors in the tests and that it's pulled into a variable named `focused` in the code:

```javascript
                focused = bindingValue.focus || ko.observable(null);
```

I say let's rip off the band-aid and change it.