Add flags argument to jv_parser_new()

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

jv.h

@@ -140,7 +140,7 @@ void jv_nomem_handler(jv_nomem_handler_f, void *);
 jv jv_load_file(const char *, int);
 
 struct jv_parser;
-struct jv_parser* jv_parser_new();
+struct jv_parser* jv_parser_new(int);
 void jv_parser_set_buf(struct jv_parser*, const char*, int, int);
 jv jv_parser_next(struct jv_parser*);
 void jv_parser_free(struct jv_parser*);