[FEATURE]: Add query pluck to return only single column value

### Describe what you want

Sometimes it is useful to have flat array of ids or any values:

```js
DB.select(posts.id).from(posts).then(r => r.map(v => v.id))
```

We can turn it into this:

```js
DB.pluck(posts.id).from(posts)
```

Plucking is available in many other database interfaces like ActiveRecord and Eloquent. They are especially handy when additional processing of the results is necessary or when working with multiple result sets.