Fix fenced code block syntax highlighting (#534)

Author: ANK-dev

ftplugin/markdown.vim

@@ -788,7 +788,7 @@ function! s:MarkdownHighlightSources(force)
     " Look for code blocks in the current file
     let filetypes = {}
     for line in getline(1, '$')
-        let ft = matchstr(line, '```\s*\zs[0-9A-Za-z_+-]*\ze.*')
+        let ft = matchstr(line, '\(`\{3,}\|\~\{3,}\)\s*\zs[0-9A-Za-z_+-]*\ze.*')
         if !empty(ft) && ft !~# '^\d*$' | let filetypes[ft] = 1 | endif
     endfor
     if !exists('b:mkd_known_filetypes')
@@ -819,8 +819,10 @@ function! s:MarkdownHighlightSources(force)
             else
                 let include = '@' . toupper(filetype)
             endif
-            let command = 'syntax region %s matchgroup=%s start="^\s*```\s*%s.*$" matchgroup=%s end="\s*```$" keepend contains=%s%s'
-            execute printf(command, group, startgroup, ft, endgroup, include, has('conceal') && get(g:, 'vim_markdown_conceal', 1) && get(g:, 'vim_markdown_conceal_code_blocks', 1) ? ' concealends' : '')
+            let command_backtick = 'syntax region %s matchgroup=%s start="^\s*`\{3,}\s*%s.*$" matchgroup=%s end="\s*`\{3,}$" keepend contains=%s%s'
+            let command_tilde    = 'syntax region %s matchgroup=%s start="^\s*\~\{3,}\s*%s.*$" matchgroup=%s end="\s*\~\{3,}$" keepend contains=%s%s'
+            execute printf(command_backtick, group, startgroup, ft, endgroup, include, has('conceal') && get(g:, 'vim_markdown_conceal', 1) && get(g:, 'vim_markdown_conceal_code_blocks', 1) ? ' concealends' : '')
+            execute printf(command_tilde,    group, startgroup, ft, endgroup, include, has('conceal') && get(g:, 'vim_markdown_conceal', 1) && get(g:, 'vim_markdown_conceal_code_blocks', 1) ? ' concealends' : '')
             execute printf('syntax cluster mkdNonListItem add=%s', group)
 
             let b:mkd_known_filetypes[ft] = 1