Add mfa command

[ericzbeard]

This command will get a session token using an MFA device and update environment settings (or overwrite a profile in the credentials file).

rain mfa <device-name> <code> <profile-name>

If you set up a virtual MFA device called phone, and your current token is 000 000, and you want to overwrite the default profile in your credentials file:

rain mfa phone 000000 default

This will call the GetSessionToken API and write the credentials to ~.aws/credentials.