Improve hooks for per-user customization.

- Add ~/.vimuser as outside-the-tree default location for per-user
  configuration files.  Fall back to ~/.vim/user if not found.  This
  allows for easier out-of-tree source control of per-user files.

- Move default location for "-before.vim" and "-after.vim" hooks, from
  $VIMFILES/$VIMUSER-{before,after}.vim to
  $VIMFILES/$VIMUSER/vimrc-{before,after}.vim.  In this way, the entire
  per-user tree $VIMFILES/$VIMUSER may be easily source-controlled as a
  unit.