请增加IPv6 本地监听地址的可选配置

[Yifei0727]

• Shadowsocks is an open source project for noneprofit. If you bought the service from a provider, please contact them. / 影梭（Shadowsocks）是一个开源非盈利项目，不提供任何托管服务。如果你是从服务提供商购买的服务，请联系他们
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• Or search from Issue Board / 或在Issue Board中搜索 https://github.com/shadowsocks/shadowsocks-windows/issues?utf8=%E2%9C%93&q=is%3Aissue
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Shadowsocks version / 影梭版本

4.1.6

Environment(Operating system, .NET Framework, etc) / 使用环境（操作系统，.NET Framework等）

1809 17763.437 64bit

Steps you have tried / 操作步骤

请增加本地配置
localHost 让我能指定地址 例如 [::1]

What did you expect to see? / 期望的结果

C:\Users\XXX>netstat -nao|find "1080"
  TCP    [::1]:1080         0.0.0.0:0              LISTENING       1868
  UDP    [::1]:1080         *:*                                    1868
C:\Users\Rui>telnet [::1] 1080
连接成功

What did you see instead? / 实际结果

C:\Users\XXX>netstat -nao|find "1080"
  TCP    127.0.0.1:1080         0.0.0.0:0              LISTENING       1868
  UDP    127.0.0.1:1080         *:*                                    1868

C:\Users\XXX>telnet [::1] 1080
正在连接[::1]...无法打开到主机的连接。 在端口 1080: 连接失败

Config and error log in detail (with all sensitive info masked) / 配置文件和日志文件（请隐去敏感信息）

[Yifei0727]

如果可以，后续在UI 界面增加一个 复选框用于影响配置 是否启用IPv6

1. 未勾选 默认，仅使用IPv4
2. 半勾选 仅使用IPv6
3. 全勾选 采用双栈（IPv4&IPv6) 【未实现】

[celeron533]

Mark IPv6 as experimental function. Currently, only allow "advanced" users to modify the config file directly.