Add `skipXml` function to the Xeno.SAX module

[s-and-witch]

graphviz generates following SVG file header:

<?xml version="1.0" encoding="UTF-8" standalone="no"?>
<!DOCTYPE svg PUBLIC "-//W3C//DTD SVG 1.1//EN"
 "http://www.w3.org/Graphics/SVG/1.1/DTD/svg11.dtd">

So, I can't drop DOCTYPE tag without dropping ?xml tag.

That may be useful for other users, so I propose this function to be in the library.