Add support for running Cog behind an http proxy

[vanstee]

@jason-riddle hit this issue when trying to run Cog behind an http proxy. Cog should be aware of the HTTP_PROXY and HTTPS_PROXY env vars and make requests accordingly. We should also add some logging around the websocket connection to make sure the proxy supports them and prints a descriptive error message if not.

[splashx]

Any update on this?

[splashx]

Just to follow up on this one: BlakeWilliams/Elixir-Slack#94

This issue depends on upstream patching Elixir-Slack which in turn depends on upstream patching of websocket_client