change grabbing user from section to config

[jasisk]

Previously would throw if there was no corresponding ssh config (reading User off non-existent section).

Changed to read User from the parsed config.

Reproduce

$ cat $HOME/.ssh/config
# I'm a glutton for punishment and haven't configured my hosts!

$ slex --host some.cool.ip.address --user internetchampion42 uptime

EDIT: I'm pretty nubby when it comes to Go. This was my interpretation of the error and making this change fixed it. ¯_(ツ)_/¯

[crosbymichael]

LGTM

thanks!