Introduce vim/configs/pre and vim/configs/post

Just ran into this locally: I initialize Vundle in
`vim/configs/pre/vundle` but make use of it in various places
(`vim/configs/vundles`, `vim/configs/notmuch`, `vim/configs/haskell`,
and so on). This only works if Vundle is loaded before everything else.

Therefore, introduce a `vim/configs/pre` and `vim/configs/post` for the
convenience of other rc file experimenters.