Connect through proxy

[dnut]

Some servers throttle based on IP address. It would be nice if each thread could use a different proxy to connect to the server. In the config, users could just add however many proxies they want. The proxies would be assigned to download threads in a round-robin manner, allowing proxies to be reused by multiple threads if there are more threads than proxies. There would always be at least one thread that downloads without using a proxy.