Add `assume_role` command for CLI

[Luzifer]

In order to work in the context of a specific role instead of awsenv shell users might want to use awsenv role <account> <rolename> and then get credentials of that assume-role-call

http://docs.aws.amazon.com/IAM/latest/UserGuide/id_roles_use_switch-role-api.html

[ctennis]

+1

[lox]

We should collaborate! https://github.com/99designs/aws-vault

[biswajitpain]

does this assume role feature available in awsenv?