tmux: add first revision of tmux.conf

Add the first revision of .tmux.conf.

Super key is set to F12, but I'm using Caps Lock as the SuperKey by
adding below lines to .Xmodmap:

remove Lock = Caps_Lock
keysym Caps_Lock = F12

Signed-off-by: Petter Mabäcker <petter@technux.se>