Whonix support in TorBirdy Settings

Just like Jondo support got integrated into Tor Birdy, it would be nice if Whonix [1] could get integrated as well.

Mozilla Thunderbird -> Tools -> Add-ons -> Tor Birdy -> Choose an anonymization service -> Whonix

Socks Host: 192.168.0.10
Port: 9102

Doing it automatically, when the WHONIX environment variable is set.

[1] https://sourceforge.net/p/whonix/wiki/Home/