Add flag that displays the AWS CLI command generated

[robins]

Currently the AWS CLI command generated when requesting an IAM token, is shown by default.

Down the line this should go away, but it has it's use, especially for someone who's just setup her AWS CLI for this purpose.

Add a psql flag that:

1. works only when IAM authentication (-I currently) is enabled
2. displays the AWS CLI command generated

If this flag is disabled, do not display the AWS CLI command generated.