Add support for custom field

I needed support for custom field.

I modified your code and added this to feed-json/template/feed-json.php:

//custom_fields
$custom_fields = get_post_custom();
foreach($custom_fields AS $key => $value) {
	if(substr($key, 0, 1) != "_") {
		foreach($value AS $_value) {
			$_value = trim($_value);
			if($_value) {
				$single[$key]=$_value;
			}
		}
	}
}

after section // tags

Might be interesting for others

https://wordpress.org/plugins/feed-json/