update location of neovim config, update link.sh

* move ~/.nvim to ~/.config/nvim
* move ~/.nvimrc to ~/.config/nvim/init.vim
* update install/link.sh to skip already existing files
* update install/link.sh to handle creating symlinks for files in newly created config/ directory

These changes are consistent with the latest neovim install, which moves away from ~/.nvim and ~/.nvimrc

config/nvim/snippets/gitcommit.snippets

@@ -0,0 +1,6 @@
+# skip jshint checking on push
+snippet jsh
+	!skip_jshint
+# add ticket number to commit
+snippet ref
+	refs:#${1}