Implement support for pluggable transports over proxies

Currently, if we have PTs enabled we block any other kinds of proxy when we parse the torrc:

  /* Check if more than one proxy type has been enabled. */
  if (!!options->Socks4Proxy + !!options->Socks5Proxy +
      !!options->HTTPSProxy + !!options->ClientTransportPlugin > 1)
    REJECT("You have configured more than one proxy type. "
           "(Socks4Proxy|Socks5Proxy|HTTPSProxy|ClientTransportPlugin|"
           "Socks5ProxyUsername|Socks5ProxyPassword)");

During a previous dev meeting, Arturo and Nick discussed how we could enable support for pluggable transport through proxies, and Arturo wrote a proposal: https://lists.torproject.org/pipermail/tor-dev/2012-February/003318.html

We should re-evaluate the proposal and implement it.