[Enhancement]: Added vim-tmux-navigator.

Added vim-tmux-navigator for seamless movement between vim and tmux splits using the same `Ctrl-h,l,j,k` as you would to navigate vim-only.

Installed locally and working like a charm. I don't believe it conflicts with any existing keybindings, but adding @skwp for verification.

Pertains to feature request #515 (@ianks @stevenbarragan).