New json function similar to Hive's json_tuple

Hive's ```json_tuple``` allows extracting multiple keys from an input json string at once. This PR implements a similar functionality (until Presto supports table generating functions) with a new ```json_extract_multiple``` function. I didn't want to use the name "json_tuple" as in the future Presto may support table generating functions and then we can have a proper ```json_tuple``` function.