Add :syn sync linebreaks=1 to improve consistency

This was reported to fix a folding issue I can't reproduce, and also
fixes some inconsistencies such as an underline style heading failing to
highlight until a redraw.

Resolves: https://github.com/tpope/vim-markdown/issues/179

syntax/markdown.vim

@@ -51,6 +51,7 @@ if !exists('g:markdown_minlines')
   let g:markdown_minlines = 50
 endif
 execute 'syn sync minlines=' . g:markdown_minlines
+syn sync linebreaks=1
 syn case ignore
 
 syn match markdownValid '[<>]\c[a-z/$!]\@!' transparent contains=NONE