Username on CloudTrail shown as `assume-aws-role-cli`

[beyondbill]

I believe username comes from here https://github.com/jbuck/assume-aws-role/blob/671e984b7f606d18510bce32ecfe310065ec020c/bin/assume-aws-role.js#L124

To make username more meaningful, I'd like to change it to something like <IAM user who's assuming the role>-<Role name>.

I'll probably spend a bit time next week to get a PR out for this.

[jbuck]

Oh yeah, that'd be a good change to add. Looking forward to it!