Instead of storing sensitive information in your hexo config, you can instead set environment variables for your S3 configuration.
Requires that the environment variables `AWS_KEY` and `AWS_SECRET` be set.
This PR allows someone to define a `region` key in `_config.yml` so that upload works correctly via `s3-sync`.
Why? [`s3-sync`](https://github.com/hughsk/s3-sync) uses [`knox`](https://github.com/LearnBoost/knox) to handle S3 commands. Unless your are uploading to `us-standard`, the upload will fail because the S3 url won't be correct.
