Raw JSON string element

[Chlumsky]

If certain parts of the JSON tree don't have a static structure or we don't care about their structure but want to preserve it, it might be useful to provide a special string type to store the subtree without parsing it, and writing it into output JSON as-is. This would also allow users to use another JSON parser for these portions or delay the parsing until it's requested. There could be an option to preserve / strip whitespace formatting for these portions.