couple logging fixes

- log is global array so log can be viewed any time
- log gets written to a defined location ~/.vim-vundle/vundle.log
- TODO: write log changes as they happen

autoload/vundle/installer.vim

@@ -201,7 +201,6 @@ func! s:system(cmd) abort
 endf
 
 func! s:log(str) abort
-  if !exists('g:vundle_log') | let g:vundle_log = [] | endif
   call add(g:vundle_log, a:str)
   return a:str
 endf