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.

[FedorLap2006]

Thanks for the contribution!

P.S. While ideally the voice (see the comment above) related changes should have been implemented here, I'll be adding them in a separate PR, since you have disabled maintainer edits on the branch.

[uWynell]

Sorry for my inactivity, I hadn't even logged into my account since this PR.

[FedorLap2006]

No problem. Additionally, as you can see, I've adressed it in the PR mentioned above.