HTTP proxy support was added in #366, but the proxy configuration only takes into account the proxy host and port. We are running in an environment that requires authenticating to the proxy, and as far as I understood, axios doesn't support that. We are using the standard `http_proxy` and `https_proxy` environment variables with the credentials in the URL, like `http://proxy-user:proxy-password@proxy-host:proxy-port`, and passing that works as expected with e.g. [superagent-proxy](https://github.com/TooTallNate/superagent-proxy).

So I'm requesting to have support for HTTP proxy authentication, preferably even taking the credentials automatically from HTTP proxy environment variables.

lib/adapters/http.js
if (config.maxRedirects) {
        options.maxRedirects = config.maxRedirects;
      }
      transport = isHttps ? httpsFollow : httpFollow;
    }
    
    var req = transport.request(options, function handleResponse(res) {
      if (aborted) return;
      
      clearTimeout(timer);
      timer = null;
      
      var stream = res;
      switch (res.headers['content-encoding']) {
      
      case 'gzip':
      case 'compress':
      case 'deflate':
        
        stream = stream.pipe(zlib.createUnzip());
        
        delete res.headers['content-encoding'];
        break;
      }
      var response = {
        status: res.statusCode,
        statusText: res.statusMessage,
        headers: res.headers,
        config: config,
        request: req