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/markdown.snippets

@@ -0,0 +1,3 @@
+snippet code
+	\`\`\`${1}
+	\`\`\`