Support informal example/program listing region that enables formatting of the contained text

The current 'code' format does not allow bold highlighting (for example). However it maps to a programlisting element in docbook that does enable contained text to be formatted.

When dealing with DSLs it is useful to be able to highlight keywords that are being explained in other text.

I tried the 'panel' format, which does allow contained text formatting, and was ok in pdf but did not have a border in the html.