Allow specifying the bind address of managed proxies

Tor instructs managed proxies to bind to a random port the first time they operate. It then stores that port in its state file, and the next time it spawns the same proxy it instructs it to bind to that port.

Currently, bridge operators don't have a way to specify the TCP port that their managed proxies will use to bind, and people rely on hacks, like editing the state file, to achieve this. People need specific bind addresses because of port forwarding and because they might want to have their obfsproxy bind to a different IP address than their INADDR_ANY.

We should think about adding an optional configuration parameter to allow bind address customization. This can be: a) An optional parameter in the ServerTransportPlugin line. b) A new torrc line called ServerTransportBindaddr or ServerTransportPluginBindaddr.

I think I'm drifting towards 'b'. It's easier to implement and in the future we might also want ServerTransportPluginStateDirectory (etc.) and I'm not sure if we want to polute the ServerTransportPlugin line that much.