[Feature request] Pick up region from `AWS_DEFAULT_REGION` or `AWS_REGION`

I always have to set the region explicitly or else I would get following errors:

```
Decryption failed: unable to decrypt parameter: MissingRegion: could not find region configuration
```

Despite of the presence of `AWS_DEFAULT_REGION` and `AWS_REGION` environment variables, it's seems to me ejsonkms never pick either of them up.