Add Transproxy mode from Shadowsocks app.

[Anderhar]

Currently, NekoBox offers a choice of two operating modes: VPN and SOCKS5/HTTP proxy, the scope of which is quite limited. But Shadowsocks app for Android has a third, the most flexible mode Transproxy, which provides users of rooted devices with many demanded options (via iptables), like tunneling native clients of VPN services or proxying proxy-incompatible apps without occupying system VPN. This is a very smart override to root support that is definitely lacking in apps like NekoBox.