Update fence logic to handle nested fences (per CommonMark spec; no handling of indentation).

http://spec.commonmark.org/0.22/#fenced-code-blocks

lib/rules.js

@@ -40,12 +40,17 @@ function filterTokens(params, type, callback) {
 function forEachLine(params, callback) {
   if (!params.forEachLine) {
     var lineMetadata = new Array(params.lines.length);
+    var fenceStart = null;
     var inFence = false;
     // Find fenced code by pattern (parser ignores "``` close fence")
     params.lines.forEach(function forLine(line, lineIndex) {
       var metadata = 0;
-      if (/^(```|~~~)/.test(line)) {
+      var match = /^(`{3,}|~{3,})/.exec(line);
+      var fence = match && match[1];
+      if (fence &&
+          (!inFence || (fence.substr(0, fenceStart.length) === fenceStart))) {
         metadata = inFence ? -2 : 2;
+        fenceStart = inFence ? null : fence;
         inFence = !inFence;
       } else if (inFence) {
         metadata = 1;

test/fenced_code_with_nesting.md

@@ -0,0 +1,73 @@
+# header
+
+text
+```fence {MD031}
+code
+``` {MD031}
+text
+~~~fence {MD031}
+code
+~~~ {MD031}
+text
+```fence {MD031}
+~~~fence
+code
+~~~
+``` {MD031}
+text
+~~~fence {MD031}
+```fence
+code
+```
+~~~ {MD031}
+text
+```fence {MD031}
+
+~~~fence
+code
+~~~
+
+``` {MD031}
+text
+~~~fence {MD031}
+
+```fence
+code
+```
+
+~~~ {MD031}
+text
+```fence {MD031}
+code
+~~~
+``` {MD031}
+text
+~~~fence {MD031}
+code
+```
+~~~ {MD031}
+text
+````fence {MD031}
+```fence
+code
+```
+```` {MD031}
+text
+~~~~fence {MD031}
+~~~fence
+code
+~~~
+~~~~ {MD031}
+text
+````fence {MD031}
+```fence
+code
+```
+````` {MD031}
+text
+~~~~fence {MD031}
+~~~fence
+code
+~~~
+~~~~~ {MD031}
+text