Description:
------------
One of the appeals of JSON is that it is human readable.  However, json_encode produces output that is not particularly manageable by a human - little more so than PHP's own serialization.  DOM, for example, has a formatOutput option that nicely indents the resulting XML.  A similar option for json_encode would be extremely helpful.

Reproduce code:
---------------
$obj = new stdClass;
$obj->field1 = "hello";
$obj->field2 = "world";
$obj->field3 = array("key" => "value", "level2" => array(1,2,3));
echo json_encode($obj, JSON::FORMAT_OUTPUT);

//flags are an implementation example

Expected result:
----------------
{
	"field1":"hello",
	"field2":"world",
	"field3":
	{
		"key":"value",
		"level2": [1,2,3]
	}
}

Actual result:
--------------
{"field1":"hello","field2":"world","field3":{"key":"value","level2":[1,2,3]}}