Configure Vagrant to use Polipo on the OS X host as an HTTP(S) caching proxy

.vagrant.d-Vagrantfile

# this saves us from both hard-coding an IP, and using a non-bridged IP for the VM.
iface = `ifconfig -l`.match(/(vbox|vmnet.*?) /)[1]
local_ip = `ifconfig #{iface}`.match(/inet (.*?) /)[1]

Vagrant.configure("2") do |config|
  if Vagrant.has_plugin?("vagrant-proxyconf")
    if `lsof -i |egrep -e '^polipo.*TCP \\*:8123.*LISTEN'`.to_s.empty?
      $stderr.puts "lsof doesn't see polipo listening on *:8123; running without a proxy."
    elsif local_ip
      config.proxy.http     = "http://#{local_ip}:8123/"
      config.proxy.https    = "http://#{local_ip}:8123/"
      config.proxy.no_proxy = "localhost,127.0.0.1,.example.com"
    end
  end
end

Not exactly bug-free or ironclad.