highlight.js emits warning if no language is set on source block

[mojavelinux]

If no language is specified on a source block, and the source-highlighter attribute is set to highlight.js, the highlight.js emits the following warning into the browser console:

Could not find the language 'none', did you forget to load/include a language module?
Falling back to no-highlight mode for this block.

Here's an example:

[source]
----
no lang
----

The warning happens because the language none is not mapped to plaintext. To fix it, either add the mapping for none or don't pass blocks with the language none to highlight.js.

[mojavelinux]

Backport the fix for this issue to the v2.0.x branch.

[mojavelinux]

Another solution is to add the CSS class language-plaintext so that highlight.js resolves the language to plaintext, which is mapped.

<div class="listingblock">
<div class="content">
<pre class="highlightjs highlight"><code class="language-plaintext language-none hljs">no lang</code></pre>
</div>
</div>
</div>

[mojavelinux]

Resolved by d5c9ab5 and backported.