Add "bind" method to link to observables

```typescript
export function bind<T, K extends keyof T>(observable: ObservableVariable<T>, key: K): ObservableVariable<T[K]> {
    const observableOfProp = Observable(observable.value[key]);
    observable.subscribe((value: T) => observableOfProp.value = value[key]);
    observableOfProp.subscribe((value) => observable.value[key] = value);
    return observableOfProp;
}
```