Introduce autorolestargetname to allow for custom target IAM roles

[nmasur]

Instead of hardcoding GhostBusterTargetAccountRole, allow for an optional role name. This is helpful for Control Tower users, since all accounts include the aws-controltower-ReadOnlyExecutionRole role for the Audit account.

Fixes #14 .

I haven't spent much time on this pull request. I just came up with something and introduced it as an example.