Support for the linenos option on highlighted code

The added examples in `docs/index-test.md` extend the previous examplees of highlighting, documenting the required inout.

_includes/fix_linenos.html

@@ -0,0 +1,33 @@
+{% comment -%}
+Fixes the HTML for highlighted code with linenos.
+
+Derived from the workaround provided by Dmitry Hrabrov (DeXP) at
+https://github.com/penibelst/jekyll-compress-html/issues/71#issuecomment-188144901
+
+Explanation:
+
+The HTML produced by Rouge with the linenos option matches CSS `code table`.
+Jekyll (<= 4.1.1) always wraps the highlighted HTML with `pre`, which is 
+unnecessary and non-conforming, and leads to validation error reports.
+The fix removes the `pre` tags around `_code` whenever it contains the pattern
+`<table class="rouge-table">`. This change avoids validation errors, and
+allows the use of [Jekyll layout for compressing HTML](http://jch.penibelst.de),
+which relies on `pre` tags not being nested. 
+
+Usage:
+
+{% capture fix_linenos_code %}{% highlight AnyLanguage linenos %}
+Some code
+{% endhighlight %}{% endcapture %}{% include fix_linenos.html %}{{ fix_linenos_code }}
+
+Caveats:
+
+The above does not work when `Some code` happens to contain the matched string 
+`<table class="rouge-table">`.
+
+{%- endcomment %}
+
+{% if fix_linenos_code contains '<table class="rouge-table">' %}
+    {% assign fix_linenos_code = fix_linenos_code | replace: "<pre><code", "<code" %}
+    {% assign fix_linenos_code = fix_linenos_code | replace: "</code></pre>", "</code>" %}
+{% endif %}