Add `Dialer` property to `Session`

[uWynell]

A little attempt to make this pull request. Why would I need such an update? For example, if I would be able to pass custom Dialer, then I could use proxy (click).

dialSocksProxy, _:= proxy.SOCKS5("tcp", "proxy_ip", nil, proxy.Direct)
s, _:= discordgo.New("token")
s.Dialer = &websocket.Dialer{Dial: dialSocksProxy.Dial}

Sorry if I'm wrong. I'll be very happy to hear where exactly I made a mistake, if there's one.