Issue Connecting to RTM and WS behind Proxy

[odininon]

Using Hedwig behind a firewall does not connect to the Slack RTM api.

Issue:

When connecting, hedwig will receive a {:error, :nxdomain} message.

Requested Usage:
Add configuration options for proxy and accompanying settings.

Examples:
config :alfred,
Alfred.Robot,
adapter: Hedwig.Adapters.Slack,
name: "aflred",
aka: "/",
token: System.get_env("SLACK_API_TOKEN"),
rooms: [],
proxy: System.get_env("HTTP_PROXY"),
ssl_options: [insecure: true, ssl_options: [protocol: 'tlsv1.2']]
responders: [
{Hedwig.Responders.Help, []},
{Hedwig.Responders.Ping, []}
]