Trailing tabs after closing fence marker

[rlidwka]

Test case:

$ cat test.md | sed 's/\t/<TAB>/g'
```
foo
```<TAB>
bar

Commonmark.js doesn't close fence because there's a trailing tab there:

$ ~/commonmark/bin/commonmark ./test.md 
<pre><code>foo
```	
bar
</code></pre>

Cmark disagrees:

$ ~/cmark/build/src/cmark ./test.md 
<pre><code>foo
</code></pre>
<p>bar</p>

There is no test in the spec for this exact case. But there is a similar case for thematic breaks (example 11) where trailing tabs are expanded to spaces:

$ echo -e ' -\t-\t-\t' | ~/commonmark/bin/commonmark 
<hr />

Also, consider the following case (imagine if user appends tabs everywhere):

```js<TAB>
foo<TAB>
```<TAB>
bar<TAB>

If line 1 is considered fence opener (as it does in both implementations), surely line 3 should be considered a closer?

---

Original issue is opened as markdown-it#388, which behavior is the same as cmark in this case.