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
port: parsed.port,
      path: buildURL(parsed.path, config.params, config.paramsSerializer).replace(/^\?/, ''),
      method: config.method,
      headers: headers,
      agent: agent,
      auth: auth
    };
    var proxy = config.proxy;
    if (!proxy) {
      var proxyEnv = protocol.slice(0, -1) + '_proxy';
      var proxyUrl = process.env[proxyEnv] || process.env[proxyEnv.toUpperCase()];
      if (proxyUrl) {
        var parsedProxyUrl = url.parse(proxyUrl);
        proxy = {
          host: parsedProxyUrl.hostname,
          port: parsedProxyUrl.port
        };
      }
    }
    if (proxy) {
      options.host = proxy.host;
      options.hostname = proxy.host;
      options.headers.host = parsed.hostname + (parsed.port ? ':' + parsed.port : '');
      options.port = proxy.port;
      options.path = protocol + '//' + parsed.hostname + (parsed.port ? ':' + parsed.port : '') + options.path;
    }
    var transport;
    if (config.maxRedirects === 0) {
      transport = isHttps ? https : http;
    } else {