Markdown wrap settings are hard to override

I could't find a clean way to override the default setting `textwidth=72` for Markdown files, which is defined in `filetypes.vim`. I tried adding the following to `.vimrc.after`:

    au FileType markdown set textwidth=0

But that setting gets overwritten on `BufEnter` by a function in `filetype.vim`. It's not possible to redefine that function in `.vimrc.after` because it's local to `filetype.vim`.

`filetype.vim` should write the wrap settings on `FileType markdown` rather than on `BufEnter` so that `.vimrc.after` can override those settings by declaring additional autocmds on `FileType markdown`.