Update MD040/fenced-code-language to add allowed_languages parameter (fixes #610).

test/md040-allowed_languages.md

@@ -0,0 +1,45 @@
+# md040-allowed_languages.md
+
+Code block with `html` not in allowed_languages:
+
+```html
+<h1>markdownlint</h1> {MD040:5}
+```
+
+Code block with `css` not in allowed_languages:
+
+```css
+body {}  {MD040:11}
+```
+
+Code block with `js` in allowed_languages:
+
+```js
+console.log('markdownlint')
+```
+
+Code block with `scss` in allowed_languages:
+
+```scss
+body {
+  h1 {
+    color: red;
+  }
+}
+```
+
+Code block with `md` in allowed_languages:
+
+```MD
+hello md
+```
+
+Code block with `TS` in allowed_languages:
+
+```ts
+body {
+  h1 {
+    color: red;
+  }
+}
+```