Improve tag filtering performance

This patch improve performance issue when displaying a page containing long list of link to other pages.

* Tags.process() has been re-written to avoid numerous parsing of the html document in is_preformated?(). The unit test written went from 20 secondes down to less than 2 seconds (less than one on my machine)
* Page#formated_data has been cached to avoid rendering the page multiple time before displaying.