Add folding support

Closes #10.

ftplugin/markdown.vim

@@ -19,4 +19,32 @@ else
   let b:undo_ftplugin = "setl cms< com< fo< flp<"
 endif
 
+function! MarkdownFold()
+  let line = getline(v:lnum)
+
+  " Regular headers
+  let depth = match(line, '\(^#\+\)\@<=\( .*$\)\@=')
+  if depth > 0
+    return ">" . depth
+  endif
+
+  " Setext style headings
+  let nextline = getline(v:lnum + 1)
+  if (line =~ '^.\+$') && (nextline =~ '^=\+$')
+    return ">1"
+  endif
+
+  if (line =~ '^.\+$') && (nextline =~ '^-\+$')
+    return ">2"
+  endif
+
+  return "="
+endfunction
+
+if has("folding") && exists("g:markdown_folding")
+  setlocal foldexpr=MarkdownFold()
+  setlocal foldmethod=expr
+  let b:undo_ftplugin .= " foldexpr< foldmethod<"
+endif
+
 " vim:set sw=2: