update linkify regexp

based on @kylewm's in https://github.com/kylewm/redwind/blob/863989d48b97a85a1c1a92c6d79753d2fbb70775/redwind/util.py#L39

the main differences are:
* supports unicode characters
* requires scheme unless the URL starts with 'www.'
* tries to ignore URLs in HTML/XML attribute values
* styled a bit differently using re.VERBOSE

thanks again @kylewm!