Add SOCKS5/HTTP proxy support

[ChaserZ98]

Developers under networks with strict firewall rules may not have access to discord server directly. The twilight-http-proxy might provide a possible solution to set up a http proxy to provide access to discord RESTful api. But that would require extra setup cost and it doesn't give a solution to websocket access.

A less costy and more flexible solution would be providing an option for developers to use their own HTTP/SOCKS5 proxy url and make HTTP/websocket connection through that url.

The following crates might be useful in implementing this feature.

• hyper-proxy
• hyper-socks2