Feature Request: json_extend, jsonb_extend

It would be nice if PostgreSQL has *_extend (similar to $.extend from
jQuery or _.extend from UnderscoreJs) for JSON/JSONB columns, so we could
write it like this:

UPDATE tablename
SET jsoncolumn = json_extend(jsoncolumn, $2)
WHERE id = $1;

instead of this:

UPDATE tablename
SET data = (
SELECT json_object_agg(key, value)::jsonb
FROM
( SELECT * FROM jsonb_each( SELECT data FROM tablename WHERE id=$1 )
UNION ALL
SELECT * FROM jsonb_each( $2 )
) x1
) WHERE id=$1;