add previous window toggle for tmux

Just like in screen.

tmux/tmux.conf

@@ -52,6 +52,7 @@ bind-key C-d detach-client
 # Window Movement
 bind-key -r C-h previous-window
 bind-key -r C-l next-window
+bind-key C-a last-window
 bind-key -n M-1 select-window -t 1
 bind-key -n M-2 select-window -t 2
 bind-key -n M-3 select-window -t 3