Fenced code block info string should not contain fence marker

Fix https://github.com/markdown-it/markdown-it/issues/301

lib/rules_block/fence.js

@@ -28,7 +28,7 @@ module.exports = function fence(state, startLine, endLine, silent) {
   markup = state.src.slice(mem, pos);
   params = state.src.slice(pos, max);
 
-  if (params.indexOf('`') >= 0) { return false; }
+  if (params.indexOf(String.fromCharCode(marker)) >= 0) { return false; }
 
   // Since start is found, we can report success here in validation mode
   if (silent) { return true; }

test/fixtures/markdown-it/commonmark_extras.txt

@@ -149,6 +149,16 @@ bar
 </ul>
 .
 
+Info string in fenced code block can't contain marker used for the fence
+.
+~~~test~
+
+~~~test`
+.
+<p>~~~test~</p>
+<pre><code class="test`"></code></pre>
+.
+
 Coverage. Directive can terminate paragraph.
 .
 a