[plugin] add ag.vim
[plugin] prevent repeated loading for google-python-style
reformat vimrc, put folding markers to make management easier

* remove deprecated files
* update README.md
* seperate filetype related settings to filetype.vimrc
* reconsider 'tab' stuff, don't enable 'expandtab' for all filetypes,
  only enable for specific filetypes
* need "unimpaired" plugin for "bubble lines" to work
* re-organize vimrc, set folding markers to make management easier