Support SOCKS5 proxies

[adamhotep]

It would be nice to have a config item like socks_proxy or socks5_proxy to support SOCKS proxies.

At the moment, even commands like tsocks mpv http://server/test.webm fail to work, and though curl --socks5 localhost:1080 http://server/test.webm |mpv --cache=512 works, you can't seek since mpv isn't controlling the network stream.

This SO question explains the issue farther.