Build a free-form config definition with default values with Symfony\Component\Config

config.php

<?php
// $builder is an instance of Symfony\Component\Config\Definition\Builder\ArrayNodeDefinition
$builder
  ->children()
    ->arrayNode('params')
      ->info('Arbitrary key => value configuration')
      ->useAttributeAsKey('name')
      ->prototype('scalar')->end()
    ->end()
  ->end();

/* When dumped as YAML I would like my config to look something like this:
 *
 * params:
 *     key1: value1
 *     key2: value2
 *
 * Then a user can add the following config:
 *
 * params:
 *     key2: some value
 *     key3: value3
 *
 * Which would result in the following configuration:
 *
 * params:
 *     key1: value1
 *     key2: some value
 *     key3: value3
 */