attributes on <Extensions>

[Will Norris <will@willnorris.com>]

I don't think we actually talked about it... do we want to allow  
arbitrary attributes on the <Extensions> element like we do everything  
else?  Currently, the schema does not allow for this.  Personally, I  
don't see any harm in it.  But to be fair, nor do I really see much  
value in it for extension authors.

-will