Pass along region information

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.
Allow the user of Environment Variables

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.