New default TLS options

[bigben93]

I tested default settings of lua-http server with testssl command.
The worst problems:

Testing protocols via sockets except NPN+ALPN

SSLv2 not offered (OK)
SSLv3 not offered (OK)
TLS 1 offered (deprecated)
TLS 1.1 offered (deprecated)
TLS 1.2 offered (OK)
TLS 1.3 offered (OK): final
NPN/SPDY not offered
ALPN/HTTP2 h2, http/1.1 (offered)

and

Testing vulnerabilities
[...]
Secure Client-Initiated Renegotiation VULNERABLE (NOT ok), DoS threat
[...]

To fix these problems HTTPS server must be run with additional TLS flags: OP_NO_TLSv1, OP_NO_TLSv1_1, OP_NO_RENEGOTIATION.

I think it would be a good idea to provide better security "out of the box".

[daurnimator]

See #217