[u104][vim] New feature: auto-save-load vim session.

1. All session files is saved in $VIM_SESSION.
   ($VIM_SESSION is set in .bashrc and default is ~/tmp/vim/session)

2. Enter vim and session is auto-loaded; Leave vim and session is auto-saved.

3. Fix "revi" command to keep opened files but reload all .vimrc and clear all
   bookmarks.