Standardize more fields for Shadowsocks JSON

[Mygod]

Currently official doc only lists a few field, however, these fields are widely used in implementations:

• plugin and plugin_opts: see #28;
• remarks: accepted and used by both shadowsocks-windows and shadowsocks-android;

We should probably standardize these as well.

Furthermore, I should probably mention that shadowsocks-android also uses a number of other extensions as evidenced here. Maybe we should have some ways to allow for extended fields as well.

[zonyitoo]

Propose some useful fields that are used in shadowsocks-rust:

{
    "ipv6_first": true,  // Resolve domains to IPv6 first if possible, maybe we could have an "ipv6_only", too

    "udp_timeout": 5,  // Durations of seconds that a UDP associations will be kept alive
    "udp_max_associations": 100,  // Maximum number of UDP associations will be kept in server

    "dns": "8.8.8.8",  // An IP address

    "no_delay": true,  // sockopt TCP_NODELAY

    "nofile": 10240,  // rlimit file descriptor soft & hard limit in *NIX system

    "manager_address": "/tmp/shadowsocks-manager.sock",  // The address of manager
    "manager_port": 6300,  // If manager_address is an IP address, then this field sets the port that manager is listening to

    // Multiple servers
    "servers": [
        {
            "address": "127.0.0.1",  // The same as "server"
            "port": 1234, // The same as "server_port"
            "method": "plain",
            "password": "pwd",
            "timeout": 5,
            "plugin": "plugin",
            "plugin_opts": "plugin_opts"
        }
    ]
}