[Grml-devel] Add tmux config to grml-etc-core

Here's a tmux.conf for inclusion in grml-etc-core.

Since everybody in #grml seemed to agree that it was a good idea to set the prefix keybinding to C-a (instead of the default C-b), I went ahead and did just that. The other settings should be less controversial.

It would be great to have a good tmux config in grml-etc-core and while there's certainly room for improvement, I feel that this patch should be a good starting point.

Regards Sebastian