Having a selectors property

Maybe it would be good to declare selectors inside the definition of the store and pass them the store automatically, so we don't have to pass the store back when we call the selectors.

Something like this
```javascript
createStore({
list: [...],
search: '',
selectors: {
getFilteredList: ({list, search}) => list.filter(e => e.includes(search))
}
})
```
And then we could call it with
```javascipt
store.selectors.getFilteredList()
```