[Feature Request] - Proxy Support

[JonatanSalas]

Our apps, are behind a proxy network, we're using this library for one of our projects. It would be really nice, if you add support for proxy.

https://aws.amazon.com/es/blogs/developer/using-the-aws-sdk-for-javascript-from-behind-a-proxy/

var AWS = require('aws-sdk');
var proxy = require('proxy-agent');

AWS.config.update({
  httpOptions: { 
    agent: proxy('http://user:password@internal.proxy.com') 
  }
});

var s3 = new AWS.S3({region: 'us-west-2'});
s3.getObject({Bucket: 'bucket', Key: 'key'}, function (err, data) {
  console.log(err, data);
});

Would you mind if I make you a PR?