Implemented with Nginx Mainline and OpenSSL 1.1.1.
Major browsers are not ready for RFC version yet, so TLS 1.2 will be chosen.
Update on 10-12:
--with-openssl-opt='enable-ec_nistp_64_gcc_128 enable-tls1_3'
Looks like this is the key configuration to make Nginx enbale TLSv1.3.