Adding the new option "attribute"

To be compliant with HTML5 specifications, which requires that the
`lang` attribute of a `pre` tag follows the
http://tools.ietf.org/html/rfc4646 (be an ISO language code), I added a
user option `attribute` to define the name of the tag attribute to add
a special language highlighting (such as "html", "css" etc) - for
tests, I made a "demo_html5.html" from the old "demo.html"