Update src/binding/defaultBindings/options.js

Allow optionsCaption to bind to an observable.
This way the caption is not hard-coded!

In order to update the binding you still need to manually call
```javascript
y("Some new caption")
x.valueHasMutated() // to get the refresh
```

where x is the one from 
```html
<select data-bind="options: x, optionsCaption: y" />
```