added support for xlink: attribute prefix

re-submitted

The 'xlink:' attribute prefix used with inline svg is not supported, it's use is described in the spec section 8.1.2.3 Attributes, see here:
http://www.w3.org/TR/html5/syntax.html#attributes-0

The bug is evident if you remove the $$$.svg block in the example tiddler "Making curved text with SVG" on tiddlywiki.com, to make it inline svg.

there is a demo of the fix here: http://xlink.tiddlyspot.com/