Do not add the `language-` prefix when the info string already starts with `language-`

[ThomasSoeiro]

commonmark.js adds a language- prefix to the info string provided after the opening code fence, regardless if it already starts with language-.

Could commonmark.js not adding the language- prefix in this case to allow using the language- prefix in the source whitout resulting in language-language-infostring in html?

E.g., using the following source:

```language-r
x <- 1
```

```r
x <- 1
```

We get the following result:

<pre><code class="language-language-r">x &lt;- 1
</code></pre>
<pre><code class="language-r">x &lt;- 1
</code></pre>

---

The commonmark spec currently says that it "does not mandate any particular treatment of the info string". A language- prefix is added in example 142.

With pandoc --from commonmark --to html5 --no-highlight, we get:

<pre class="language-r"><code>x &lt;- 1</code></pre>
<pre class="r"><code>x &lt;- 1</code></pre>

[ThomasSoeiro]

@jgm

Thanks for the fix!

BTW, would you mind releasing a new version? There have been some useful fixes since the last stable release.

[jgm]

The changes include changes to spec.txt, so we'd need to do a new release of the spec as well. Should probably do that, but this is just to let you know why it isn't a trivial thing.