vim.vam: fix filetype detection

According to https://github.com/MarcWeber/vim-addon-manager/blob/master/doc/vim-addon-manager-additional-documentation.txt#L1310
`filetype on` must be called after all VAM plugins are initialized.
Otherwise vim set `conf` filetype for some files containing hash tags.

pkgs/misc/vim-plugins/vim-utils.nix

@@ -262,13 +262,14 @@ let
   in writeText "vimrc" ''
   " minimal setup, generated by NIX
   set nocompatible
-  filetype indent plugin on | syn on
 
   ${vamImpl}
   ${pathogenImpl}
   ${vundleImpl}
   ${neobundleImpl}
 
+  filetype indent plugin on | syn on
+
   ${customRC}
   '';