Add config for new panes/windows to be in CWD

In Tmux 1.9 new windows and panes no longer put you in your current working
directoy, they just throw you to $HOME. If you add `-c
"#{pane_current_path}"` then they will put in your current working
directory.

Source: http://unix.stackexchange.com/questions/12032/create-new-window-with-current-directory-in-tmux