Description:
------------
When trying to debug a large amount of JSON output that has been created using json_encode, it would be really nice to have an option to format the output beautifully, with some line-breaks and indenting, so the the generated JSON is easier to read.

Something a bit like the output of print_r.

Test script:
---------------
$bigarray = array();
for ($i = 0; $i < 100; $i++) {
    $bigarray['key' . $i] = 'value' . $i;
}
echo json_encode($bigarray, JSON_PRETTY_PRINT);
// Will error, because I made up the JSON_PRETTY_PRINT constant name.

Expected result:
----------------
{
    "key0":"value0",
    "key1":"value1",
    "key2":"value2",
    ...
}

Actual result:
--------------
{"key0":"value0","key1":"value1","key2":"value2",...