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
'Data after transformation must be a string, an ArrayBuffer, or a Stream',
          config
        ));
      }
      
      headers['Content-Length'] = data.length;
    }
    
    var auth = undefined;
    if (config.auth) {
      var username = config.auth.username || '';
      var password = config.auth.password || '';
      auth = username + ':' + password;
    }
    
    var parsed = url.parse(config.url);
    var protocol = parsed.protocol || 'http:';
    if (!auth && parsed.auth) {
      var urlAuth = parsed.auth.split(':');
      var urlUsername = urlAuth[0] || '';
      var urlPassword = urlAuth[1] || '';
      auth = urlUsername + ':' + urlPassword;
    }
    if (auth) {
      delete headers.Authorization;
    }
    var isHttps = protocol === 'https:';
    var agent = isHttps ? config.httpsAgent : config.httpAgent;
    var options = {
      hostname: parsed.hostname,