Add String::explode and Arrays::flatten filters

These filters work together nicely and can be used separately to cover several usecases as well.

For example:
```php
$data = ['foo' => ['a,b', 'c,d']];
list($status, $filteredParams, $error) = Filterer::filter(['foo' => [['ofScalars', [['string'], ['explode']]], ['flatten']]], $data);
var_dump($filteredParams);
//array(1) {
//  'foo' =>
//  array(4) {
//    [0] =>
//    string(1) "a"
//    [1] =>
//    string(1) "b"
//    [2] =>
//    string(1) "c"
//    [3] =>
//    string(1) "d"
//  }
//}
```