Adds ssh-config and installation-step for that

The installation is slightly different: ssh doesn't allow for splitting
up the config file: https://bugzilla.mindrot.org/show_bug.cgi?id=1585
so we copy, not link, the ssh-config file only if there is no
$HOME/.ssh/config file already.

If you know any way to tackle this a bit nicer (checking if the settings
we want to set are already there would be really nice), let me know.
Ignores the local backup folders