Configuration.php

<?php
    /**
     * @inheritDoc
     */
    public function getConfigTreeBuilder()
    {
        $treebuilder = new TreeBuilder();

        $rootNode = $treebuilder->root('');
        $rootNode
            ->children()
                ->arrayNode('targets')
                    ->prototype('scalar')->end()
                ->end()
            ->end()
        ;

        return $treebuilder;
    }

input.json

{
  "targets": ["foo", "bar"]
}