* Use AWS_VAULT as the profile if set
[aws-vault](https://github.com/99designs/aws-vault) sets
the `AWS_VAULT` env var instead of `AWS_PROFILE` when an
aws-vault session is active.
This PR adds support for reading the AWS profile value from
the `AWS_VAULT` env var, giving priority to `AWS_VAULT` if both
it and `AWS_PROFILE` are set.
* Update docs for AWS_VAULT
This updates the default AWS default symbol to include a space after the
symbol to make it a little bit more readable. Have also updated the
README to include the AWS module.
