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
module.exports = {
  tearDown: function (callback) {
    server.close();
    server = null;
    if (proxy) {
      proxy.close()
      proxy = null;
    }
    if (process.env.http_proxy) {
      delete process.env.http_proxy;
    }
    callback();
  },
  testTimeout: function (test) {
    server = http.createServer(function (req, res) {
      setTimeout(function () {
        res.end();
      }, 1000);
    }).listen(4444, function () {
      var success = false, failure = false;
      var error;
      axios.get('http://localhost:4444/', {
        timeout: 250
      }).then(function (res) {
        success = true;
      }).catch(function (err) {
        error = err;
        failure = true;
      });
      setTimeout(function () {