tmux enable mouse mode

Before tmux 2.1

set -g mode-mouse on
set -g mouse-resize-pane on
set -g mouse-select-pane on
set -g mouse-select-window on

For tmux 2.1 and later

setw -g mouse on 

Reference

https://tangledhelix.com/blog/2012/07/16/tmux-and-mouse-mode/