feat: Add whoami command

[patricksanders]

Add a weep whoami command to give users a handy and enriched alias to aws sts get-caller-identity, inspired by benkehoe/aws-whoami. Users can optionally configure SWAG to include the account name in the output.

Output with SWAG:

$ weep whoami
Role:    superAwesomeRole
Account: sweetAccount (123456789012)
ARN:     arn:aws:sts::123456789012:assumed-role/superAwesomeRole/coolSessionName
UserId:  AROAXXXXXXXXXXXXXX:coolSessionName

Output without SWAG:

$ weep whoami
Role:    superAwesomeRole
Account: 123456789012
ARN:     arn:aws:sts::123456789012:assumed-role/superAwesomeRole/coolSessionName
UserId:  AROAXXXXXXXXXXXXXX:coolSessionName

Closes #79