add proxies splash code example

Example: add proxies splash

function main(splash, args)
  local host = ''
  local port = 1111
  local user = ''
  local pass = ''
  
  splash:on_request(function()
      request:set_proxy{host, port, username=user, password=pass}
    end)
  
  splash.images_enabled = false
  assert(splash:go(splash.args.url))
  return splash:html()
end