Support for authenticated proxies

[sheerun]

From what I see this gem supports only proxies that don't require Authorization header to be set:

    def initialize(proxy_addr, proxy_port)
      timeout = ProxyFetcher.config.proxy_validation_timeout

      @http = HTTP.follow.via(proxy_addr, proxy_port.to_i).timeout(connect: timeout, read: timeout)
    end

it would be nice if there was such possibility

[nbulaj]

Hi @sheerun, nice idea, would you like to propose a PR? I'm totally for it 👍

[nbulaj]

Added here #37

Btw I don't know how we can reuse it in the Manager just because all the providers don't have such info. Only if you manually use ProxyValidator