socks5h support

Proposal

please add support for socks5h proxies

Background

export ALL_PROXY="socks5h://..."
export HTTP_PROXY="socks5h://..."
export HTTPS_PROXY="socks5h://..."

echo "GET example.com" | vegeta ...

but dns requests go to the system resolver, not to the proxy at the same time golang itself correctly uses these environment variables to make dns requests exclusively through a proxy

Workarounds

maybe add full proxy support via golang.org/x/net/proxy