Enable Tor to load Torrc from <stdin> removing the need to write it to filesystem

This ticket is to enable Tor to load Torrc from remove the need to write it.

Tor wrapper libraries such as txtorcon from meejah, today write "torrc" in temporary directory when starting Tor.

It would be good if Tor could load Torrc from standard input, that way wrapper libraries starting Tor would be able to dynamically "feed" Tor a configuration file, without having to write to filesystem.