Add script as an option to the 'sum' function

Added script as the 4th parameter to the 'sum' function. Now, it is possible to perform more complexed operations on the sum function. For example, we can do the following:
Item::es()->aggregate('Value', function($q) {
$q->sum('', null, null, ['source' => 'doc.price.value * doc.quantity.value'])->groupBy('status');
})->get()->aggregations();