Redesign command structure

[hopkinsth]

This tool is inspired by okta-aws (https://github.com/99designs/aws-vault), and I would prefer that oktad's command structure more closely followed okta-aws.

I'm thinking...

oktad exec aws ec2 describe-instances
oktad add main

and more details as I fill them out here.

I'm late to the game here, but 👍