Allow connecting to Tor using a socket file

I added the possibility of connecting to Tor using a socket file instead of a TCP port. TCP port can also now be changed without editing the plugin. The method of changing connect method, port, and path to socket file are env.connectmethod, env.port, and env.socket, respectively. Connect method defaults to 'port', port defaults to 9051, and socket defaults to /var/run/tor/control (the default location on Debian, and the default in Stem).