although vim has its own completion (C-x C-v), but it''s leak of something
vim omni supports:
* builtin command name completion.
* builtin function name completion.
* runtime command completion.
* runtime function name completion.
* g:,s:.. scope completion.
* option name completion.
* autocommand event name completion
* feature name completion.
copy ftplugin/vim/omni.vim to your ~/.vim/ftplugin/vim/omni.vim
function name: cal
command name: com!
var name: let g:
autocmd event: autocmd
option name: set
feature name: if has(
