[tmux] Only source ~/.tmux.conf.local if it exists

tmux/.tmux.conf

@@ -101,7 +101,7 @@ unbind r
 bind r source-file ~/.tmux.conf \; display 'Reloaded tmux.conf'
 
 # TODO Figure out why this is order dependent
-source-file ~/.tmux.conf.local
+if-shell "[[ -e ~/.tmux.conf.local ]]" "source-file ~/.tmux.conf.local"
 
 source-file ~/.dotfiles/src/tmux-colors-solarized/tmuxcolors-256.conf
 source-file ~/.dotfiles/src/powerline/powerline/bindings/tmux/powerline.conf