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.

test/unit/adapters/http.js
});
      });
    });
  },
  testGunzipErrorHandling: function (test) {
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'application/json;charset=utf-8');
      res.setHeader('Content-Encoding', 'gzip');
      res.end('invalid response');
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').catch(function (error) {
        test.done();
      });
    });
  },
  testUTF8: function (test) {
    var str = Array(100000).join('ж');
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end(str);
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        test.equal(res.data, str);
        test.done();
      });
    });
  },
  testBasicAuth: function (test) {
    server = http.createServer(function (req, res) {
      res.end(req.headers.authorization);