Expose shadowsocks to homebrew by Polipo

It is difficult to access python.org from China mainland so it failed when I tried brew install python. Shadowsocks provides socks proxy but Homebrew who invokes curl implicitly cannot leverage socks proxy directly. Polipo is a http proxy and can use socks proxy as an internet connection way. Fortunately it is ok to install Polipo directly from China mainland.

brew install polipo
polipo socksParentProxy=localhost:1080
export https_proxy=http://127.0.0.1:8123
export http_proxy=http://127.0.0.1:8123
brew install python