Push parsing support

[Filip Halaxa]

Support to incrementally feed the parser via an explicit method call where the pull approach of foreach cannot be used. Useful for example for curl's CURLOPT_WRITEFUNCTION or when receiving json chunks in an event loop.

Proposed usage (implicit):

$items = new PushItems(['pointer' => '/results']);

$callback = function ($jsonChunk) use ($items) {
    $items->push($jsonChunk);
    foreach($items as $item) {
        // process currently available items
    }
}

or more explicit (similar to current API):

$queue = new QueueChunks();
$items = Items::fromQueue($queue, ['pointer' => '/results']);

$callback = function ($jsonChunk) use ($items, $queue) {
    $queue->push($jsonChunk);
    foreach($items as $item) {
        // process currently available items
    }
}

Any other proposal?