Add addSelect method to Query builder

# 3.0 ORM: Add ability to select custom field with all default fields

This resolves https://github.com/cakephp/cakephp/issues/4360

Use `addSelect` method to add additional select fields to the query, not overwrite defaults and associations.

```PHP
// Example. This will load Articles as usuall but add myField method with appropriate value
$query = $this -> Articles ->  addSelect( [ 'myField' => 'field + 20' ] );
```