add a class to links created by urlfilter

I'd like to suggest the attached patch applied to urlfilter.

It adds a class 'urlfilter-generated-ok' or 'urlfilter-generated-trunc' to the links.

The motivation is for usage in print style sheets: I print the value of the href attribute of links, but for links generated by urlfilter this is not nessecary (unless they are truncated of course).