Add username to log output for each executed command.

[asemt]

This pull request contains a really small change which only adds the the SSH username to the log output (where username and host are separated by the @ symbol). The actual username value is the one parsed from the User ssh-config directive. This way it's easier to spot under which user the command was actually executed on the remote host (without consulting the ~/.ssh/config file).

An example run would now look like (where username would be the value of the User directive for a host called example.org in the ~/.ssh/config file):

$ ./slex --host example.org --agent uptime
INFO[0000] Session complete from username@example.org:22
 17:34:09 up 183 days, 47 min,  2 users,  load average: 2.59, 2.71, 2.72 

[crosbymichael]

LGTM

Thanks!

[asemt]

Thank you for merging it (and so fast)!

Best,
Andreas