Revert ":BundleInstall is gone for now"

what was I thinking...?!

This reverts commit 035cc6ecba.

autoload/vundle.vim

@@ -7,6 +7,9 @@
 com! -nargs=+         Bundle
    \ call vundle#config#bundle(<args>)
 
+com! -nargs=? -bang -complete=custom,vundle#scripts#complete BundleInstall
+   \ call vundle#installer#install('!' == '<bang>', <q-args>)
+
 com! -nargs=? -bang   BundleClean
    \ call vundle#installer#clean('!' == '<bang>')
 

autoload/vundle/scripts.vim

@@ -10,6 +10,10 @@ func! vundle#scripts#all(bang, ...)
   endif
 endf
 
+func! vundle#scripts#complete(a,c,d)
+  return join(s:load_scripts(0),"\n")
+endf
+
 func! vundle#scripts#install() abort
   let line = substitute(substitute(getline('.'), '\s*Bundle\s*','','g'), "'",'','g')
   call vundle#installer#install(0, line)