Add flags argument to jv_parser_new()

For extensibility.  We might add streaming parser options, even binary
JSON encoding options.

jv_parse.c

@@ -370,7 +370,7 @@ static pfunc scan(struct jv_parser* p, char ch, jv* out) {
   return answer;
 }
 
-struct jv_parser* jv_parser_new() {
+struct jv_parser* jv_parser_new(int flags) {
   struct jv_parser* p = jv_mem_alloc(sizeof(struct jv_parser));
   parser_init(p);
   return p;