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
response.end(body + '1234');
          });
        });
      }).listen(4000, function() {
        // 
        process.env.http_proxy = 'http://localhost:4000/';
        axios.get('http://localhost:4444/').then(function(res) {
          test.equal(res.data, '45671234', 'should use proxy set by process.env.http_proxy');
          test.done();
        });
      });
    });
  },
  testCancel: function(test) {
    var source = axios.CancelToken.source();
    server = http.createServer(function (req, res) {
      // 
      source.cancel('Operation has been canceled.');
    }).listen(4444, function() {
      axios.get('http://localhost:4444/', {
        cancelToken: source.token
      }).catch(function (thrown) {
        test.ok(thrown instanceof axios.Cancel, 'Promise must be rejected with a Cancel obejct');
        test.equal(thrown.message, 'Operation has been canceled.');
        test.done();
      });
    });
  }
};