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
test.equal(success, false, 'request should not succeed');
        test.equal(failure, true, 'request should fail');
        test.equal(error.code, 'ECONNABORTED');
        test.equal(error.message, 'timeout of 250ms exceeded');
        test.done();
      }, 300);
    });
  },
  testJSON: function (test) {
    var data = {
      firstName: 'Fred',
      lastName: 'Flintstone',
      emailAddr: 'fred@example.com'
    };
    server = http.createServer(function (req, res) {
      res.setHeader('Content-Type', 'application/json;charset=utf-8');
      res.end(JSON.stringify(data));
    }).listen(4444, function () {
      axios.get('http://localhost:4444/').then(function (res) {
        test.deepEqual(res.data, data);
        test.done();
      });
    });
  },
  testRedirect: function (test) {
    var str = 'test response';
    server = http.createServer(function (req, res) {
      var parsed = url.parse(req.url);
      if (parsed.pathname === '/one') {
        res.setHeader('Location', '/two');