vim,tmux:plugs: Add vim-tmux-navigator

[vim-tmux-navigator][1]:

> Seamless navigation between tmux panes and vim splits

Add the hybrid vim/tmux plugin `vim-tmux-navigator` to switch between
tmux panes and vim splits with `C-[hjkl]`.

Add vim-tmux-navigator as a submodule in tmux/plugins and create a
symlink from vim/pack/plugins/start.

Get rid of similar switching keymappings in the vim and tmux configs.

[1]: https://github.com/christoomey/vim-tmux-navigator

.gitmodules

@@ -35,6 +35,9 @@
 [submodule "tpm"]
 	path = .config/tmux/plugins/tpm
 	url = https://github.com/tmux-plugins/tpm
+[submodule "vim-tmux-navigator"]
+	path = .config/tmux/plugins/vim-tmux-navigator
+	url = https://github.com/christoomey/vim-tmux-navigator
 
 # vim
 [submodule "arm-syntax-vim"]