support for websockets with ssl

[picaoao]

It is great that you added websocket support.
It would be even great if you could add websocket secure (ssl) !

[milgner]

Note that although it's not supported explicitly, since emqtt uses gun for establishing websocket connections, it is my interpretation of the code that it should automatically switch to TLS if one connects to the default port 443: https://github.com/ninenines/gun/blob/f4162604f502bc4e3805c80f303f58acef68614f/src/gun.erl#L895

[RyanSept]

I can confirm this worked for me. Thanks @milgner