Update MD040/fenced-code-language to add language_only parameter to reject extra data in info string.

test/md040-language_only.md

@@ -0,0 +1,25 @@
+# md040-language_only.md
+
+Fence code block information with leading whitespace:
+
+```  html
+<h1>markdownlint</h1> {MD040:5}
+```
+
+Fence code block information with trailing whitespace:
+
+```css 
+body {} {MD040:11} {MD009:11}
+```
+
+Fence code block information with extra data:
+
+```html version=5
+<title>MarkdownLint</title> {MD040:17}
+```
+
+Fence code block information without whitespaces and extra data:
+
+```css
+a {}
+```