Use existing session token if one exists

[janicujo]

Use an existing session_token from the source profile if one already exists.

Our specific use case is:

1. An external tool (aws-azure-login) is used to gain a temporary session token to a master account
2. awsume is used to switch between different roles in different accounts as permitted by the policies in the master account

This should help with #46 as well.

I have not had a chance to validate this change against other typical use-cases - YMMV.

[mbarneyjr]

#56 was merged, so this functionality is now implemented

Thanks for the PR!