Add option for custom endpoint

For my dev set-up I use https://github.com/localstack/localstack. In order to be able to do test deploys to my local stack I need to be able to point to a custom local AWS API endpoint. With localstack I can do e.g.: `aws --endpoint-url=http://localhost:4568 kinesis list-streams`.

I think this would be a cool feature to have for demo'ing or taking `up` for a test drive without the need for actually having an AWS account or even an active internet connection..

I will try and submit a PR for this.