Feature: Local Endpoint Global Argument

First, thanks for this utility! I noticed with `aws-shell` you can still pass in all the standard arguments for commands which includes endpoint mapping for local utility alternatives (e.g. dynamodb-local). After having used this tool frequently with some local development projects, I thought it might be useful to raise a feature request which would allow any service with local environment alternatives to be passed in via a global argument while instantiating `aws-shell`.

For example, if I am working with DynamoDB locally, I may want to use the following command to begin all interactions with a local endpoint:

`$ aws-shell --profile --endpoint-url ;`