vim-markdown/after/ftplugin/markdown.vim

" folding for Markdown headers, both styles (atx- and setex-)
" http://daringfireball.net/projects/markdown/syntax#header
"
" this code can be placed in file
"   $HOME/.vim/after/ftplugin/markdown.vim
"
" original version from Steve Losh's gist: https://gist.github.com/1038710

func! s:is_mkdCode(lnum)
    return synIDattr(synID(a:lnum, 1, 0), 'name') == 'mkdCode'
endfunc

func! s:effective_line(lnum)
    let line = getline(a:lnum)
    return (line !~ '^[=-#]' || s:is_mkdCode(a:lnum)) ? '' : line
endfunc

func! Foldexpr_markdown(lnum)
    let l2 = s:effective_line(a:lnum+1)
    if  l2 =~ '^==\+\s*'
        " next line is underlined (level 1)
        return '>1'
    elseif l2 =~ '^--\+\s*'
        " next line is underlined (level 2)
        return '>2'
    endif

    let l1 = s:effective_line(a:lnum)
    if l1 =~ '^#'
        " don't include the section title in the fold
        return '-1'
    endif

    if (a:lnum == 1)
        let l0 = ''
    else
        let l0 = s:effective_line(a:lnum-1)
    endif
    if l0 =~ '^#'
        " current line starts with hashes
        return '>'.matchend(l0, '^#\+')
    else
        " keep previous foldlevel
        return '='
    endif
endfunc


if !get(g:, "vim_markdown_folding_disabled", 0)
  setlocal foldexpr=Foldexpr_markdown(v:lnum)
  setlocal foldmethod=expr
endif