Added a way to override or define new renderers.

I needed a way to process some pages specifically. (As an exemple, I store the list of books I read as a bibliography, and I want the result HTML page to have some interesting features like block collapses, and distinct classes for comics or essays for example.)

I could have used hooks, but it would have been uselessly complicated since this is the job of the renderer. So I have added a way to add new renderers, in the spirit of what was done for hooks. In the same time, I have slightly optimised the process of searching the renderer to apply on a page. I hope you will be interested in this, or return useful comments if not.