Fix emphasis matching where delimiters are multiple of 3

This is now allowed as per commonmark 0.29 spec:
ex***amp***le

See also:
83ed53e12a

fix https://github.com/markdown-it/markdown-it/issues/561

lib/rules_inline/balance_pairs.js

@@ -23,11 +23,22 @@ module.exports = function link_pairs(state) {
           currDelim.end < 0 &&
           currDelim.level === lastDelim.level) {
 
+        var odd_match = false;
+
         // typeofs are for backward compatibility with plugins
-        var odd_match = (currDelim.close || lastDelim.open) &&
-                        typeof currDelim.length !== 'undefined' &&
-                        typeof lastDelim.length !== 'undefined' &&
-                        (currDelim.length + lastDelim.length) % 3 === 0;
+        if ((currDelim.close || lastDelim.open) &&
+            typeof currDelim.length !== 'undefined' &&
+            typeof lastDelim.length !== 'undefined') {
+
+          // from spec:
+          // sum of the lengths [...] must not be a multiple of 3
+          // unless both lengths are multiples of 3
+          if ((currDelim.length + lastDelim.length) % 3 === 0) {
+            if (currDelim.length % 3 !== 0 || lastDelim.length % 3 !== 0) {
+              odd_match = true;
+            }
+          }
+        }
 
         if (!odd_match) {
           lastDelim.jump = i - j;

test/fixtures/commonmark/bad.txt

@@ -65,31 +65,3 @@ error:
 </ol>
 
 
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-src line: 6912
-
-.
-foo***bar***baz
-.
-<p>foo<em><strong>bar</strong></em>baz</p>
-.
-
-error:
-
-<p>foo***bar***baz</p>
-
-
-~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
-src line: 6918
-
-.
-foo******bar*********baz
-.
-<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>
-.
-
-error:
-
-<p>foo******bar*********baz</p>
-
-

test/fixtures/commonmark/good.txt

@@ -5432,6 +5432,24 @@ src line: 6901
 <p><em>foo<strong>bar</strong></em></p>
 .
 
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+src line: 6912
+
+.
+foo***bar***baz
+.
+<p>foo<em><strong>bar</strong></em>baz</p>
+.
+
+~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
+src line: 6918
+
+.
+foo******bar*********baz
+.
+<p>foo<strong><strong><strong>bar</strong></strong></strong>***baz</p>
+.
+
 ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
 src line: 6927