`commonmark.js` should first evaluate character references, before interpreting “words” of the info string

[wooorm]

Here are some examples:

```	js
```
``` js
```
``` js
```
```js	
```
```js 
```
```js 
```
```js	a
```
```js a
```
```js a
```

While the spec is somewhat vague, I believe that the encoded whitespace in the first two examples, should behave like spaces, which is what happens with all the other examples, and that that is the intent of the spec.
Github does treat all these examples as js, and adds their class to it. CommonMark.js currently yields:

<pre><code></code></pre>
<pre><code></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>
<pre><code class="language-js"></code></pre>

I expect <pre><code class="language-js"></code></pre> for everything (like GH).

[jgm]

I agree that commonmark.js's behavior on the first two cases is buggy.
(cmark does the right thing here.)