Fix vim/tmux pane switching

I recently realised that the uniform tmux/vim pane switching isn't working as expected.
More specifically, I was unable to switch between vim and tmux panes with same set of keys (i.e. c-h/j/k/l). 

After checking the tmux configuration required in the [christoomey/vim-tmux-navigator](https://github.com/christoomey/vim-tmux-navigator) bundle and adding it to my tmux.conf, this issue was solved.