Enable to cancel "copy-mode" with "[" key

ref. http://blog.m4i.jp/entry/2012/02/13/155233
Enhance key bindings for controling windows and panes

Disable "x" to kill a pane with a prompt
Enable "x" and "C-x" to kill a pane without a prompt
Enable "\" to split a window vertically
Enable "'" to split a window horizontally
Modify the key bindings for the prefix key

Disable "F4" as a primary prefix key
Set "F6" to the new primary prefix key
Set "C-t" to the secondary prefix key
Merge branch 'tmux'