~/.tmux.conf: Add shortcut to do a vertical 20% split

I do this a lot, so let's automate it!

.tmux.conf

bind-key -n C-PageDown select-window -t:+1
bind-key -n C-T new-window -c "#{pane_current_path}"
bind-key -n C-S split-window -v\; resize-pane -y 20
set -g status-keys emacs
bind-key -T copy-mode-vi ? command-prompt -i -I "#{pane_search_string}" -p "(search up)" "send -X search-backward-incremental \"%%%\""
bind-key -T copy-mode-vi / command-prompt -i -I "#{pane_search_string}" -p "(search down)" "send -X search-forward-incremental \"%%%\""