vim: Update vim plugins

1. rust.vim
2. vim-markdown
vim: Add the nerdtree plugin
vimrc: Added set incsearch
vimrc: Remove the tabstop option
Remove redundant '.git' from .gitmodules
Organize vimrc

1. use 'set nofoldenable' instead of 'let g:vim_markdown_folding_disabled=1'
2. etc
vim: Add vim-airline