Use Vim's shipped Markdown

dotfiles/vim/vimrc

   augroup vimrcEx
   au!
   autocmd FileType text setlocal textwidth=78
+  autocmd BufNewFile,BufReadPost *.md set filetype=markdown
   autocmd BufReadPost *
     \ if line("'\"") > 1 && line("'\"") <= line("$") |
     \   exe "normal! g`\"" |