Read Session Token from env variables

[Lusitaniae]

Extending on #29

It's commonly to use MFA to authenticate with AWS in which case the personal IAM accounts will use a temporary session token rather than the static credentials.

https://github.com/lonelyplanet/aws-mfa

Which introduces two new env variables, which are automatically read by aws-cli and other tools interacting with AWS

AWS_SESSION_TOKEN
AWS_SECURITY_TOKEN