treat transcient bundles as configured

- "transcient bundle" - a bundle installed with :BundleInstall or interactively
  without being declared in .vimrc

- treat them the as if they were configured
- so everything works the same

- ie before this change helptags weren't generated automatically
  with `BundleInstall bundle` or with interactive mode

- it changes the way `:BundleClean` works, since it removes directories
  that aren't bundles
- FIX: `:source .vimrc` to let `:BundleClean` remove transcient bundles

- closes #99

autoload/vundle/config.vim

@@ -3,6 +3,7 @@ func! vundle#config#bundle(arg, ...)
   call s:rtp_rm_a()
   call add(g:bundles, bundle)
   call s:rtp_add_a()
+  return bundle
 endf
 
 func! vundle#config#init()