shadowsocks-libev: add nft_tcp_extra/nft_udp_extra options

shadowsocks-libev: add nft_tcp_extra/nft_udp_extra options

To add extra statement to tcp/udp forward rule, example:

```
config ss_rules 'ss_rules'
    ...
    option nft_tcp_extra 'tcp dport { 80, 443 }' # tcp only forward connections with dport 80 or 443
    option nft_udp_extra 'udp dport { 53 }' # udp only forward connections with dport 53
```

This somewhat restores the old ipt_args functionality.

Signed-off-by: Zhong Jianxin <azuwis@gmail.com>
Signed-off-by: Yousong Zhou <yszhou4tech@gmail.com>
(Amend README.md a bit)