added christoomey/vim-tmux-navigator and key maps <c-h/j/k/l> for nav…

[hophacker]

• added Plugin christoomey/vim-tmux-navigator

  This plugin is a repackaging of Mislav Marohnić's tmux-navigator configuration described 
  in this gist.  When combined with a set of tmux key bindings, the plugin will allow you to 
  navigate seamlessly between vim and tmux splits using a consistent set of hotkeys.

• added keymaps for navigation both inside panes of vim, and outside panes in tmux:

  nnoremap <silent> <c-h> :TmuxNavigateLeft<cr>
  nnoremap <silent> <c-j> :TmuxNavigateDown<cr>
  nnoremap <silent> <c-k> :TmuxNavigateUp<cr>
  nnoremap <silent> <c-l> :TmuxNavigateRight<cr>

[hophacker]

Any comment?