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
proxy: {
            host: 'localhost',
            port: 4000
          }
        }).then(function(res) {
          test.equal(res.data, '123456789', 'should pass through proxy');
          test.done();
        });
      });
    });
  },
  testHTTPProxyEnv: function(test) {
    server = http.createServer(function(req, res) {
      res.setHeader('Content-Type', 'text/html; charset=UTF-8');
      res.end('4567');
    }).listen(4444, function() {
      proxy = http.createServer(function(request, response) {
        var parsed = url.parse(request.url);
        var opts = {
          host: parsed.hostname,
          port: parsed.port,
          path: parsed.path
        };
        http.get(opts, function(res) {
          var body = '';
          res.on('data', function(data) {
            body += data;
          });
          res.on('end', function() {
            response.setHeader('Content-Type', 'text/html; charset=UTF-8');