Add Proxy Support to Tabby

I believe with limitation on internet in Iran, it will be very good to add proxy support to Tabby. Support for HTTP, HTTPS, SOCKSv4 and SOCKSv5 proxies like a browser(Firefox for example). 

It will be very helpful, so people like us can connect to servers in Iran or people in Iran can do their work using Tabby with the proxy they set on it