Adding DOM tree support to Doxia?

Hi,

While it's nice that Doxia supports stream-parsing, there are valid  
use cases for generating a DOM tree of source elements (the Blocks  
that can be found in the confluence and twiki parser implementations  
for example).

Since the confluence and twiki parsers already need these blocks, and  
since I need them too for the XWiki parser I think there's already a  
valid use case for them.

In addition, I would add the method to return this DOM tree to the  
Parser class in doxia-core (or add 2 interfaces, one for stream  
parsers and one for DOM parsers).

WDYT?

Note: In XWiki land we need to parse all pages into a DOM tree for  
performance reasons: we'll cache the DOM tree so that the rendering  
can be done faster.

Thanks
-Vincent