Improve readability of the generated file

- Add user identity (github login) to the generated file
- Add option to set the SSH_USER env variable per ssh key to help with tracking which user logged into the system (inspired by [this](http://www.screenage.de/blog/2012/02/10/how-to-log-history-and-logins-from-multiple-ssh-keys-under-one-user-account-with-puppet/))