New option "attribute" for HTML5 compliance

Hello Evgeny,

As I said in my email, here are my modifications (not a big stuff) to avoid validation error on the "lang" attribute on "pre" tags.

Commit message :
To be compliant with HTML5 specifications, which requires that the
`lang` attribute of a `pre` tag follows the
[RFC4646](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".

Thanks for your plugin, it's very useful and so light-weight (!).

P.