Prevent collapsed highlighting at large code block

Following this guidance (https://github.com/tpope/vim-markdown) while
trying to troubleshoot this issue
(masukomi/vim-markdown-folding#35).

.vimrc

@@ -57,6 +57,7 @@ set nocompatible
 if has("autocmd")
   filetype plugin indent on
 endif
+let g:markdown_minlines = 1000
 
 " Searching.
 set incsearch  " Search as characters are entered.