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
axios.get('http://localhost:4444/', {
        maxContentLength: 2000
      }).then(function (res) {
        success = true;
      }).catch(function (err) {
        error = err;
        failure = true;
      });
      setTimeout(function () {
        test.equal(success, false, 'request should not succeed');
        test.equal(failure, true, 'request should fail');
        test.equal(error.message, 'maxContentLength size of 2000 exceeded');
        test.done();
      }, 100);
    });
  },
  testStream: function(test) {
    server = http.createServer(function (req, res) {
      req.pipe(res);
    }).listen(4444, function () {
      axios.post('http://localhost:4444/',
        fs.createReadStream(__filename), {
        responseType: 'stream'
      }).then(function (res) {
        var stream = res.data;
        var string = '';
        stream.on('data', function (chunk) {
          string += chunk.toString('utf8');
        });
        stream.on('end', function () {