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
reject(enhanceError(err, config));
    });
    
    if (config.timeout && !timer) {
      timer = setTimeout(function handleRequestTimeout() {
        req.abort();
        reject(createError('timeout of ' + config.timeout + 'ms exceeded', config, 'ECONNABORTED'));
        aborted = true;
      }, config.timeout);
    }
    if (config.cancelToken) {
      
      config.cancelToken.promise.then(function onCanceled(cancel) {
        if (aborted) {
          return;
        }
        req.abort();
        reject(cancel);
        aborted = true;
      });
    }
    
    if (utils.isStream(data)) {
      data.pipe(req);
    } else {
      req.end(data);
    }
  });
};