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.responseType === 'stream') {
        response.data = stream;
        settle(resolve, reject, response);
      } else {
        var responseBuffer = [];
        stream.on('data', function handleStreamData(chunk) {
          responseBuffer.push(chunk);
          
          if (config.maxContentLength > -1 && Buffer.concat(responseBuffer).length > config.maxContentLength) {
            reject(createError('maxContentLength size of ' + config.maxContentLength + ' exceeded', config));
          }
        });
        stream.on('error', function handleStreamError(err) {
          if (aborted) return;
          reject(enhanceError(err, config));
        });
        stream.on('end', function handleStreamEnd() {
          var responseData = Buffer.concat(responseBuffer);
          if (config.responseType !== 'arraybuffer') {
            responseData = responseData.toString('utf8');
          }
          response.data = responseData;
          settle(resolve, reject, response);
        });
      }
    });
    
    req.on('error', function handleRequestError(err) {
      if (aborted) return;