Enhanced XDXF support (second attempt)

This time, I have redone it right, according to Goldendict coding conventions. Please, pull my code into the main repo.

PS. I am extremely bad at C++, but I'm utterly eager to improve XDXF support. May I ask a guy who can code c++ to help me with a couple of questions?

The same way DICT_TYPE type (XDXF / Stardict) is passed from xdxf.cc to xdxf2html.cc convert() function, I want to pass dictionary format (logical / visual) and standard_version (all this is contain in <xdxf lang_from="pol" lang_to="rus" format="logical" standard_version="30">). Please help me improve XDXF support!

For you guys this is a matter of 2 minutes (just to pass an argument to a function), but for me it's an hours or trying to understand what I did wrong =(