Enable setting torbutton.use_nontor_proxy with an environment variable

I've been working with some Whonix people to try and get a working i2p browser into Whonix, and for obvious reasons, Tor Browser was the only acceptable basis. My goal was to accomplish this without changing Tor Browser at all, but unfortunately that was not to be. In order to use i2p, I need to be able to tell Torbutton to set use_nontor_proxy=true. I have a fork on github where I successfully did this already https://github.com/eyedeekay/torbutton/commit/3879775737a640a78e4cbe99605ac22d7b201a0a which I am using to test it.

Trac:
Username: eyedeekay