Can `Builder::attribute_filter` be made to not require `static`?

[pitdicker]

Similar to #176 I would like to write a callback for Builder::attribute_filter which doesn't have a 'static lifetime.

But I just can't figure out if this is possible. This is my current attempt: pitdicker@7c69882

And this is used to test the functionality (for fixing absolute URLs with spaces or other syntax errors):

struct CleanUrl<'a>(Option<&'a ammonia::Url>);

impl<'a> AttributeFilter<'a> for CleanUrl<'a>
{
    fn filter<'val>(&self, element: &str, attribute: &str, value: &'val str) -> Option<Cow<'val, str>> {
        // Fix invalid links image urls
        match (element, attribute) {
            ("a", "href") => {
                eprintln!("href: {}", value);
                Some(match self.0 {
                    Some(base) => base.join(value),
                    None => ammonia::Url::parse(value),
                }.map(|u| Cow::Owned(u.into())).unwrap_or(value.into()))
            }
            ("img", "src") => {
                eprintln!("src: {}", value);
                Some(match self.0 {
                    Some(base) => base.join(value),
                    None => ammonia::Url::parse(value),
                }.map(|u| Cow::Owned(u.into())).unwrap_or(value.into()))
            }
            _ => Some(value.into())
        }
    }
}