Always prefer language- prefixed classes.

If a block's class cstring ontains language-something it is a clear
indication that the user wants this class to be treated as a
language name. We should take that into account and not look at
other classes present.

This also allows to disable highlighting in blocks with classes
we recognize as languages but that might have different meaning:

    <code class="c language-text">..</code>

src/highlight.js

@@ -44,27 +44,24 @@ https://highlightjs.org/
   }
 
   function blockLanguage(block) {
-    var i, language, length,
+    var i, match, length,
         classes = block.className + ' ';
 
     classes += block.parentNode ? block.parentNode.className : '';
-    classes = classes.split(/\s+/);
-
-    // try to find a supported language or no-highlight class
-    for(i = 0, length = classes.length; i < length; i++) {
-      language = classes[i].replace(/^lang(uage)?-/, '');
 
-      if(getLanguage(language) || isNotHighlighted(language)) {
-        return language;
-      }
+    // language-* takes precedence over non-prefixed class names and
+    match = /\blang(?:uage)?-([\w-]+)\b/.exec(classes);
+    if (match) {
+      return getLanguage(match[1]) ? match[1] : 'no-highlight';
     }
 
-    // return 'no-highlight' if there is an unsupported lang(uage)-prefixed class
+    classes = classes.split(/\s+/);
     for(i = 0, length = classes.length; i < length; i++) {
-      if (/^lang(uage)?-/.test(classes[i])) {
-        return 'no-highlight';
+      if(getLanguage(classes[i]) || isNotHighlighted(classes[i])) {
+        return classes[i];
       }
     }
+
   }
 
   function inherit(parent, obj) {

test/index.html

@@ -23,10 +23,6 @@
 <pre><code class="lang-python">def add_one(x):
     return x + 1</code></pre>
 
-<!-- html 5 style language class plus unsupported language class -->
-<pre><code class="language-foo language-python">for x in [1, 2, 3]:
-  count(x)</code></pre>
-
 </div>
 
 <div id="custom-markup">
@@ -84,6 +80,9 @@
 <!-- unsupported shortened language-prefixed class -->
 <pre><code class="lang-foo">for x in [1, 2, 3]: count(x)</code></pre>
 
+<!-- unsupported prefixed language and supported unprefixed language -->
+<pre><code class="python language-foo">for x in [1, 2, 3]: count(x)</code></pre>
+
 </div>
 
 <!-- sub-languages -->

test/special/noHighlight.js

@@ -47,6 +47,13 @@ describe('no highlighting', function() {
       actual.should.equal(expected);
   });
 
+  it('should keep block unchanged (unsupported prefixed language)', function() {
+      var expected = 'for x in [1, 2, 3]: count(x)',
+          actual   = document.querySelector('#no-highlight .python.language-foo').innerHTML;
+
+      actual.should.equal(expected);
+  });
+
   it('should skip pre tags without a child code tag', function() {
     var expected = 'Computer output',
         actual   = document.querySelector('pre samp').innerHTML;