Theory:
	When Vim start, it first process .vimrc file; Then scans all directories in ~/.vim for plugins contained in pack/*/start 

1、install vim plugins and auto loading:
	(1) create directory:
		mkdir -p ~/.vim/pack/vendor/start
	(2) install plugin
		git clone --depth 1 https://github.com/preservim/nerdtree.git ~/.vim/pack/vendor/start/nerdtree
		
2、install vim plugins and manually loading:
	(1) create directory:
		mkdir -p ~/.vim/pack/vendor/opt
	(2) install plugin 
		git clone --depth 1 https://github.com/preservim/nerdtree.git ~/.vim/pack/vendor/opt/nerdtree
	(3) manually loading in vim command mode
		:packadd nerdtree