torrc
Socks5Proxy host[:port]
References
https://www.torproject.org/docs/tor-manual.html.en
http://superuser.com/questions/565722/how-to-config-tor-to-use-a-http-socks-proxy
https://svn.torproject.org/svn/tor/tags/tor-0_0_9_5/src/config/torrc.sample.in